This function intercepts HTTP requests made through
httr and serves mock
file responses instead. It is what
with_mock_api() does, minus the
automatic disabling of mocking when the context finishes.
Nothing; called for its side effects.
Note that you in order to resume normal request behavior, you will need to
stop_mocking() yourself---this function does not clean up after itself
as 'with_mock_api` does.