fix(BaseClient): Prevent user agent mutation

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
Resolves #9421 by removing the mutation (bad bad bad). We instead make use of the spread operator.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[vercel]

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2 Ignored Deployments

Name	Status	Preview	Comments	Updated (UTC)
discord-js	⬜️ Ignored (Inspect)	Visit Preview		Apr 28, 2023 5:30pm
discord-js-guide	⬜️ Ignored (Inspect)	Visit Preview		Apr 28, 2023 5:30pm

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	100
🟢 Accessibility	100
🟢 Best practices	100
🟢 SEO	100
🟠 PWA	70

Lighthouse ran on https://discord-js-git-fix-appending-user-agent-discordjs.vercel.app/

[codecov]

Codecov Report

Merging #9425 (403f37b) into main (36216c0) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 403f37b differs from pull request most recent head 5d4d167. Consider uploading reports for the commit 5d4d167 to get more accurate results

@@            Coverage Diff             @@
##             main    #9425      +/-   ##
==========================================
- Coverage   59.30%   59.26%   -0.04%     
==========================================
  Files         222      222              
  Lines       14630    14581      -49     
  Branches     1257     1251       -6     
==========================================
- Hits         8676     8642      -34     
+ Misses       5914     5899      -15     
  Partials       40       40              

Flag	Coverage Δ
brokers	66.51% <ø> (ø)
builders	98.57% <ø> (ø)
collection	99.88% <ø> (ø)
guide	0.00% <ø> (ø)
next	∅ <ø> (∅)
proxy	81.08% <ø> (+5.08%)	⬆️
rest	91.89% <ø> (ø)
util	98.42% <ø> (ø)
utilities	100.00% <ø> (ø)
voice	63.47% <ø> (ø)
website	0.00% <ø> (ø)
ws	55.56% <ø> (ø)

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see 3 files with indirect coverage changes

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Resolved.