[1.26] Don't send 'User-Agent' twice if header is binary

[sethmlarson]

Backport of PR #2047

[codecov]

Codecov Report

Merging #2048 (b1d3a76) into 1.26.x (d2e3d03) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            1.26.x     #2048   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         2297      2297           
=========================================
  Hits          2297      2297           

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d2e3d03...b1d3a76. Read the comment docs.