Remove redundant to_sym in Mockery

- Object#respond_to? has always accepted either string or symbol
  interchangeably (at least from Ruby v1.8.4 onwards).

- Mocha::ClassMethods__method_exists__? accepts either string or symbol
  since it uses Module#public_method_defined?,
  #protected_method_defined? & private_method_defined? which have
  accepted either string or symbol interchangeably (at least from Ruby
  v1.8.4 onwards), c.f. 9705028.

lib/mocha/mockery.rb

@@ -118,10 +118,9 @@ def mocha_inspect
     end
 
     def on_stubbing(object, method)
-      method = method.to_sym
       signature_proc = lambda { "#{object.mocha_inspect}.#{method}" }
       check(:stubbing_non_existent_method, 'non-existent method', signature_proc) do
-        !(object.stubba_class.__method_exists__?(method, true) || object.respond_to?(method.to_sym))
+        !(object.stubba_class.__method_exists__?(method, true) || object.respond_to?(method))
       end
       check(:stubbing_non_public_method, 'non-public method', signature_proc) do
         object.stubba_class.__method_exists__?(method, false)