Add last_request method to httpretty wrapper

Author: ajhodges

src/shipchain_common/test_utils/httpretty_asserter.py

@@ -47,6 +47,9 @@ def register_uri(self,  # noqa
     def latest_requests(self):
         return self.mock.calls
 
+    def last_request(self):
+        return self.mock.calls[-1] if self.mock.calls else None
+
     def reset(self):
         self.mock._calls.reset()  # pylint:disable=protected-access