Fix: NoMethodError on decrement when output is non-TTY

Why This Change Is Necessary
========================================================================

- [x] Bug Fix
- [ ] New Feature

Calling decrement when the output is not TTY enabled causes a no method
error as `bar_update_string` returns `nil` where a string is expected.

How These Changes Address the Issue
========================================================================

This change ensures a string is returned so there is no runtime error
and adds a test case.

Side Effects Caused By This Change
========================================================================

- [ ] This Causes a Breaking Change
- [x] This Does Not Cause Any Known Side Effects

Checklist
========================================================================

- [x] I have run `rubocop` against the codebase
- [x] I have added tests to cover my changes
- [x] All new and existing tests passed

------------------------------------------------------------------------
Github Issue URLs:
  * #147

.rubocop.yml

@@ -3,6 +3,9 @@ inherit_from:
 
 # Put your project-specifc Rubocop config here
 
+Layout/ClassStructure:
+  Enabled: false
+
 Layout/ClosingParenthesisIndentation:
   Enabled: false
 

lib/ruby-progressbar/calculators/length.rb

@@ -39,7 +39,7 @@ def length_override=(other)
 
   # This code was copied and modified from Rake, available under MIT-LICENSE
   # Copyright (c) 2003, 2004 Jim Weirich
-  # rubocop:disable Lint/RescueWithoutErrorClass
+  # rubocop:disable Style/RescueStandardError
   def terminal_width
     return 80 unless unix?
 
@@ -48,7 +48,7 @@ def terminal_width
   rescue
     80
   end
-  # rubocop:enable Lint/RescueWithoutErrorClass
+  # rubocop:enable Style/RescueStandardError
 
   # rubocop:disable Lint/DuplicateMethods
   begin
@@ -68,6 +68,7 @@ def dynamic_width
       dynamic_width_via_system_calls
     end
   end
+  # rubocop:enable Lint/DuplicateMethods
 
   def dynamic_width_via_output_stream_object
     _rows, columns = output.winsize

lib/ruby-progressbar/outputs/non_tty.rb

@@ -22,7 +22,7 @@ def bar_update_string
     output_string           = formatted_string[last_update_length..-1]
     self.last_update_length = formatted_string.length
 
-    output_string
+    output_string.to_s
   end
 
   def default_format

lib/ruby-progressbar/refinements/enumerator.rb

@@ -2,7 +2,6 @@ class  ProgressBar
 module Refinements
 module Enumerator
 refine ::Enumerator do
-  # rubocop:disable Metrics/BlockNesting
   def with_progressbar(options = {}, &block)
     chain = ::Enumerator.new do |yielder|
       progress_bar = ProgressBar.create(options.merge(:starting_at => 0, :total => size))
@@ -18,7 +17,6 @@ def with_progressbar(options = {}, &block)
 
     chain.each(&block)
   end
-  # rubocop:enable Metrics/BlockNesting
 end
 end
 end

lib/ruby-progressbar/time.rb

@@ -1,3 +1,4 @@
+# rubocop:disable Style/InlineComment
 class   ProgressBar
 class   Time
   TIME_MOCKING_LIBRARY_METHODS = [
@@ -28,3 +29,4 @@ def unmocked_time_method
   attr_accessor :time
 end
 end
+# rubocop:enable Style/InlineComment

spec/ruby-progressbar/base_spec.rb

@@ -499,6 +499,36 @@ class    ProgressBar
     end
   end
 
+  context 'decrementing the bar' do
+    it 'displays the bar with the correct progress' do
+      progressbar = ProgressBar::Base.new(:output        => output,
+                                          :length        => 20,
+                                          :starting_at   => 1,
+                                          :total         => 6,
+                                          :throttle_rate => 0.0)
+
+      progressbar.decrement
+
+      expect(output_string).to eql "                    \r" \
+                                   "Progress: |=       |\r" \
+                                   "Progress: |        |\r"
+    end
+
+    context 'for non-TTY enabled devices' do
+      it 'does nothing' do
+        progressbar = ProgressBar::Base.new(:output        => non_tty_output,
+                                            :length        => 20,
+                                            :starting_at   => 2,
+                                            :total         => 6,
+                                            :throttle_rate => 0.0)
+        progressbar.decrement
+
+        expect(non_tty_output_string).to eql "\n" \
+                                           "Progress: |=="
+      end
+    end
+  end
+
   it 'can be converted into a hash' do
     Timecop.freeze(::Time.utc(2012, 7, 26, 18, 0, 0))