Fix "@disable_warnings" spec warning (#416)

When running rspec specs on the ruby-grape project with warnings enabled
this warning is printed hundreds of times.

This change fixes the warning.

Also, turn on warnings when running rspec to see when this happens.

CHANGELOG.md

@@ -36,6 +36,7 @@ scheme are considered to be bugs.
 
 ### Miscellaneous
 
+* [#416](https://github.com/intridea/hashie/pull/416): Fix `warning: instance variable @disable_warnings not initialized` - [@axfcampos](https://github.com/axfcampos).
 * Your contribution here.
 
 ## [3.5.5] - 2017-02-24

lib/hashie/mash.rb

@@ -83,7 +83,7 @@ module Hashie
     # @api semipublic
     # @return [Boolean]
     def self.disable_warnings?
-      !!@disable_warnings
+      @disable_warnings ||= false
     end
 
     # Inheritance hook that sets class configuration when inherited.

spec/spec_helper.rb

@@ -19,4 +19,5 @@ RSpec.configure do |config|
   config.expect_with :rspec do |expect|
     expect.syntax = :expect
   end
+  config.warnings = true
 end