Pass nil to net_http as proxy parameter not to use environment variables
Change else to elsif so that regular users just get the ENV variables.
Fix blocking test introduced by @d9e4839

If the block yielded to the user exited early #fetch would try to check
in a connection to a pool which could only hold one connection and would
block forever.

Now the request portion of #fetch has been extracted to perform_request
which avoids the problem.