Do not overwrite custom headers if user agent isn't specified

[Cadair]

Fixes #85

[codecov]

Codecov Report

Merging #89 (07ff582) into main (8652805) will increase coverage by 0.15%.
The diff coverage is n/a.

❗ Current head 07ff582 differs from pull request most recent head 5ca4c56. Consider uploading reports for the commit 5ca4c56 to get more accurate results

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   91.82%   91.98%   +0.15%     
==========================================
  Files           4        4              
  Lines         477      474       -3     
==========================================
- Hits          438      436       -2     
+ Misses         39       38       -1     

Impacted Files	Coverage Δ
parfive/downloader.py	95.57% <ø> (+0.29%)	⬆️

---

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 8ce3935...5ca4c56. Read the comment docs.