Add `useMutation` optimistic response test (#3264)
Add `useMutation` optimistic response test (#3264)

Optimistic response handling is already verified through the
`Mutation` component test suite (which ultimately calls into
`useMutation`), but we'll add a `useMutation` specific test to
help validate #3217.

Fixes #3217.
apollostack/react-apollo,apollographql/react-apollo,apollostack/react-apollo,apollographql/react-apollo