Strip whitespace for proxy urls, test for same. #4613

[ghost]

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I'm not sure this is the right place to do this operation, but it does solve the problem. Or at least it passes the test that I wrote for it.

Quite likely I'm not reading them correctly, but as far as I can tell the other test methods related to proxies don't test something like a requests.get() call with the expectation of a successful result.

I found a ton of tests for error cases, though, and modeled the test for this situation on those, but it seems like there would be a better way.

[codecov-io]

Codecov Report

Merging #4627 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #4627   +/-   ##
=======================================
  Coverage   66.62%   66.62%           
=======================================
  Files          15       15           
  Lines        1564     1564           
=======================================
  Hits         1042     1042           
  Misses        522      522

Impacted Files	Coverage Δ
requests/adapters.py	70.18% <100%> (ø)	:arrow_up:

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