Fix HTTPS proxy handling

[alga]

When HTTPS requests were made through a proxy, the proxy host name and port ended up in the cassette URLs.

In this PR, I extend the Proxy fixture to handle the CONNECT method, reproduce the bug and fix it.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3a5ff1c) 92.32% compared to head (53faa64) 92.34%.
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #809      +/-   ##
==========================================
+ Coverage   92.32%   92.34%   +0.01%     
==========================================
  Files          27       27              
  Lines        1811     1815       +4     
  Branches      338      339       +1     
==========================================
+ Hits         1672     1676       +4     
  Misses         92       92              
  Partials       47       47              

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[graingert]

Previously you set the host of the proxy server to an explicit 127.0.0.x IP. Is that needed?

[alga]

Previously you set the host of the proxy server to an explicit 127.0.0.x IP. Is that needed?

Not strictly speaking, just the proxy having a different IP than httpbin was convenient for debugging and made the test a bit more precise. If the URLs vary just by the port number, there can conceivably be a regression uncaught by tests if the host selection logic breaks.

I reverted it to minimize the random changes in the diff when resolving a conflict. Do you think it's worth putting it back?

[alga]

Ping @graingert, so is this good to go?