[1.26] Don't compare bytes and str in putheader()

[sethmlarson]

Backport of #2141

[codecov]

Codecov Report

Merging #2142 (3b106a8) into 1.26.x (b89158f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           1.26.x    #2142   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          25       25           
  Lines        2303     2303           
=======================================
  Hits         2301     2301           
  Misses          2        2           

Impacted Files	Coverage Δ
src/urllib3/connection.py	100.00% <100.00%> (ø)

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[sethmlarson]

Looks like macOS 2.7 is failing (I believe this is a failure we've seen before) and PyPy on Travis is failing with cryptography-related errors.

[pquentin]

We want to backport https://github.com/urllib3/urllib3/pull/2035/files for the source_address issue. Never seen AttributeError: '_cffi_backend.CData' object is not a context manager (no __enter__/__exit__ method), though.

Both are unrelated to the PR

[sethmlarson]

Going to merge and release since all the failures are unrelated. CI will have to be fixed up separately.