WIP: Implicitly set responder on partial mocks

This relates to #149 and #531.

All the tests seem to pass, but it would be worth doing a bit more
thinking about the change in Mock#check_responder_responds_to where we
now set include_all to true for the call to Object#respond_to?.

Also it's only really worth doing this if the investigation in #531
means that it's definitely worthwhile. It would probably also be worth
doing some spiking on the responder-related solution proposed in #149.

lib/mocha/class_methods.rb

@@ -12,7 +12,7 @@ def initialize(klass)
 
       def mocha(instantiate = true)
         if instantiate
-          @mocha ||= Mocha::Mockery.instance.mock_impersonating_any_instance_of(@stubba_object)
+          @mocha ||= Mocha::Mockery.instance.mock_impersonating_any_instance_of(@stubba_object).responds_like_instance_of(@stubba_object)
         else
           defined?(@mocha) ? @mocha : nil
         end

lib/mocha/mock.rb

@@ -381,7 +381,7 @@ def raise_unexpected_invocation_error(invocation, matching_expectation)
     end
 
     def check_responder_responds_to(symbol)
-      if @responder && !@responder.respond_to?(symbol) # rubocop:disable Style/GuardClause
+      if @responder && !@responder.respond_to?(symbol, true) # rubocop:disable Style/GuardClause
         raise NoMethodError, "undefined method `#{symbol}' for #{mocha_inspect} which responds like #{@responder.mocha_inspect}"
       end
     end

lib/mocha/object_methods.rb

@@ -14,7 +14,7 @@ module ObjectMethods
     # @private
     def mocha(instantiate = true)
       if instantiate
-        @mocha ||= Mocha::Mockery.instance.mock_impersonating(self)
+        @mocha ||= Mocha::Mockery.instance.mock_impersonating(self).responds_like(self)
       else
         defined?(@mocha) ? @mocha : nil
       end