Extract handle_method_call from method_missing

Continues the refactoring started in #549 on the road to better keyword
args matching. This is part of a refactor sketched out in #544.

The key change is to pass arguments and the block directly instead of
re-splatting them. That way, we would only need to set ruby2_keywords at
the edges, instead of with each splat (for contrast, see the initial
spike in #534, where ruby2_keywords had to be set at multiple layers).

The extracting of handle_method_call is not strictly necessary, but it
makes it clearer that method_missing is just one of the possible entry
points for a method call.

lib/mocha/mock.rb

@@ -309,6 +309,11 @@ def all_expectations
 
     # @private
     def method_missing(symbol, *arguments, &block) # rubocop:disable Style/MethodMissingSuper
+      handle_method_call(symbol, arguments, block)
+    end
+
+    # @private
+    def handle_method_call(symbol, arguments, block)
       check_expiry
       check_responder_responds_to(symbol)
       invocation = Invocation.new(self, symbol, arguments, block)

lib/mocha/stubbed_method.rb

@@ -57,7 +57,7 @@ def define_new_method
       self_in_scope = self
       method_name_in_scope = method_name
       stub_method_owner.send(:define_method, method_name) do |*args, &block|
-        self_in_scope.mock.method_missing(method_name_in_scope, *args, &block)
+        self_in_scope.mock.handle_method_call(method_name_in_scope, args, block)
       end
       retain_original_visibility(stub_method_owner)
     end