fix: correct the User-Agent casing

[childish-sambino]

No description provided.

[codecov]

Codecov Report

Merging #881 into master will not change coverage by %.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master     #881   +/-   ##
=======================================
  Coverage   82.48%   82.48%           
=======================================
  Files          70       70           
  Lines        2312     2312           
  Branches      340      340           
=======================================
  Hits         1907     1907           
  Misses        206      206           
  Partials      199      199           

Impacted Files	Coverage Δ
sendgrid/sendgrid.py	83.33% <83.33%> (ø)

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