* Fix the default_connection proxy test
The assignment to the mock connection:
`Faraday.default_connection = mock_connection`
was persisting throughout the tests.
If the random number generator is unlucky the following error occurs:
\#<Double "Connection"> was originally created in one example but has leaked into another example and can no longer be used. rspec-mocks' doubles are designed to only last for one example, and you need to create a new one in each example you wish to use it for.
This commit fixes the issue by explicitly setting the default_connection
back to nil in an "after" block after the tests have run.
Subsequent calls to `default_connection` within the test suite then
behave as previously expected irrespective of the random test ordering.
