Add a warning when using mocking without cache (#261)
Add a warning when using mocking without cache (#261)

If HttpMock or HttpMockSequence is used in a test without the cache
being filled first, a strange KeyError will be thrown. This happens
on e.g. Travis CI. Unfortunately this behavior is used in the tests;
this patch adds a warning for those unfortunate enough to encounter
it.

See also: #208
googleapis/google-api-python-client,googleapis/google-api-python-client