Change BaseResponse action parsing to be a bit more generic.

Some service like SWF have action headers like
com.amazonaws.swf.service.model.SimpleWorkflowService.<action>. By using
-1 instead of 1, we can accomodate both scenarios.

moto/core/responses.py

@@ -142,7 +142,7 @@ class BaseResponse(_TemplateEnvironmentMixin):
             # Headers are case-insensitive. Probably a better way to do this.
             match = self.headers.get('x-amz-target') or self.headers.get('X-Amz-Target')
             if match:
-                action = match.split(".")[1]
+                action = match.split(".")[-1]
 
         action = camelcase_to_underscores(action)
         method_names = method_names_from_class(self.__class__)