Build User-Agent string only once

[moorereason]

No description provided.

[codecov]

Codecov Report

Merging #221 into v2 will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##               v2     #221   +/-   ##
=======================================
  Coverage   96.18%   96.18%           
=======================================
  Files           9        9           
  Lines        1127     1127           
=======================================
  Hits         1084     1084           
  Misses         23       23           
  Partials       20       20

Impacted Files	Coverage Δ
client.go	97.15% <ø> (ø)	⬆️
redirect.go	94.11% <0%> (ø)	⬆️
middleware.go	91.28% <100%> (ø)	⬆️

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[jeevatkm]

@moorereason Thank you for the PR 😄