Update Ruby tests

- Add instructions to run tests
- Ensure minitest is a development dependency
- Add tests for blank emails

ruby-mailchecker.gemspec

@@ -12,4 +12,5 @@ Gem::Specification.new do |spec|
 
   spec.files         = ['platform/ruby/mail_checker.rb', 'platform/ruby/ruby-mailchecker.rb']
   spec.require_paths = ['platform/ruby']
+  spec.add_development_dependency 'minitest'
 end

test/platform.ruby.test.rb

@@ -1,8 +1,10 @@
+# Run tests from the repository root directory:
+# $ bundle install && bundle exec ruby test/platform.ruby.test.rb
 require 'minitest/autorun'
 
 require_relative '../platform/ruby/mail_checker'
 
-class TestMailChecker < MiniTest::Unit::TestCase
+class TestMailChecker < MiniTest::Test
   def valid!(email)
     assert_equal true, MailChecker(email)
   end
@@ -21,13 +23,15 @@ def test_return_true_if_valid
   end
 
   def test_return_false_if_email_invalid
+    invalid!('')
+    invalid!('  ')
     invalid!('plopplop.com')
     invalid!('my+ok@ok=plop.com')
     invalid!('my,ok@ok.plop.com')
-    invalid!('ok@tmail.com')
   end
 
   def test_return_false_if_throwable_domain
+    invalid!('ok@tmail.com')
     invalid!('ok@33mail.com')
     invalid!('ok@ok.33mail.com')
     invalid!('ok@guerrillamailblock.com')