Fix infinite recursive call when mounting endpoint with common ancestor

Parent and child classes were pointing to the same inheritable setting
creating an infinite loop when retrieving values.

lib/grape/dsl/settings.rb

@@ -168,10 +168,10 @@ def within_namespace(&_block)
       # Builds the current class :inheritable_setting. If available, it returns the superclass's :inheritable_setting.
       # Otherwise, a clean :inheritable_setting is returned.
       def build_top_level_setting
-        if defined?(superclass) && superclass.respond_to?(:inheritable_setting) && superclass != Grape::API
-          superclass.inheritable_setting
-        else
-          Grape::Util::InheritableSetting.new
+        Grape::Util::InheritableSetting.new.tap do |setting|
+          if defined?(superclass) && superclass.respond_to?(:inheritable_setting) && superclass != Grape::API
+            setting.inherit_from superclass.inheritable_setting
+          end
         end
       end
     end

spec/grape/loading_spec.rb

@@ -1,8 +1,14 @@
 require 'spec_helper'
 
 describe Grape::API do
-  let(:jobs_api) do
+  let(:base_api) do
     Class.new(Grape::API) do
+      format :json
+    end
+  end
+
+  let(:jobs_api) do
+    Class.new(base_api) do
       namespace :one do
         namespace :two do
           namespace :three do
@@ -18,15 +24,15 @@
 
   let(:combined_api) do
     JobsApi = jobs_api
-    Class.new(Grape::API) do
+    Class.new(base_api) do
       version :v1, using: :accept_version_header, cascade: true
       mount JobsApi
     end
   end
 
   subject do
     CombinedApi = combined_api
-    Class.new(Grape::API) do
+    Class.new(base_api) do
       format :json
       mount CombinedApi => '/'
     end