Ensure IndifferentAccess is injected after merge

During non-destructive merges (i.e. `#merge`), the `IndifferentAccess` mixin was
failing to inject itself into the resulting Hash. This caused a `NoMethodError`
to be thrown when attempting to perform the action.

Now, we properly inject the mixin when necessary so the `#merge` method works.

.rubocop_todo.yml

@@ -1,6 +1,6 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2017-02-24 07:11:40 -0600 using RuboCop version 0.34.2.
+# on 2018-02-04 16:33:11 -0600 using RuboCop version 0.34.2.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
@@ -34,6 +34,11 @@ Metrics/LineLength:
 Metrics/MethodLength:
   Max: 28
 
+# Offense count: 1
+# Configuration parameters: CountComments.
+Metrics/ModuleLength:
+  Max: 102
+
 # Offense count: 6
 Metrics/PerceivedComplexity:
   Max: 10
@@ -48,7 +53,7 @@ Style/CaseEquality:
 Style/Documentation:
   Enabled: false
 
-# Offense count: 11
+# Offense count: 10
 Style/DoubleNegation:
   Exclude:
     - 'lib/hashie/dash.rb'

CHANGELOG.md

@@ -29,6 +29,7 @@ scheme are considered to be bugs.
 ### Fixed
 
 * [#435](https://github.com/intridea/hashie/pull/435): Mash `default_proc`s are now propagated down to nested sub-Hashes - [@michaelherold](https://github.com/michaelherold).
+* [#436](https://github.com/intridea/hashie/pull/436): Ensure that `Hashie::Extensions::IndifferentAccess` injects itself after a non-destructive merge - [@michaelherold](https://github.com/michaelherold).
 * Your contribution here.
 
 ### Security

lib/hashie/extensions/indifferent_access.rb

@@ -133,8 +133,10 @@ module Hashie
         self
       end
 
-      def merge(*)
-        super.convert!
+      def merge(*args)
+        result = super
+        IndifferentAccess.inject!(result) if hash_lacking_indifference?(result)
+        result.convert!
       end
 
       def merge!(*)

spec/hashie/extensions/indifferent_access_spec.rb

@@ -49,6 +49,19 @@ describe Hashie::Extensions::IndifferentAccess do
       expect(merged_hash[:cat]).to eq('meow')
       expect(merged_hash['cat']).to eq('meow')
     end
+
+    it 'injects the resulting new Hash with IndifferentAccess' do
+      hash = IndifferentHashWithMergeInitializer.new(
+        :cat => 'meow',
+        'dog' => { name: 'Mango', sound: 'woof' }
+      )
+
+      dog = hash[:dog]
+      merged = dog.merge(foo: 'bar')
+
+      expect(merged[:foo]).to eq('bar')
+      expect(merged['foo']).to eq('bar')
+    end
   end
 
   describe '#merge!' do