fix($http): do not modify the config object passed into $http short methods

Update $http's createShortMethods and createShortMethodsWithData
to extend an empty object instead of the passed-in config.
Previously, since $http was extending the passed-in config,
the changes to the config object persisted even after the call to $http's
get/post/etc. returned. This causes unexpected behavior if that
config object is reused in subsequent calls to $http.
The existing test in httpSpec was not properly testing this situation.

Closes: #11764