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

[sethmlarson]

Previously if b"user-agent" was sent for a non-chunked request then we'd emit multiple User-Agent headers, this closes that gap. This commit will need to be backported to 1.26

[codecov]

Codecov Report

Merging #2047 (ebe7051) into master (00f1769) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2047   +/-   ##
=========================================
  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)

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