fix: correctly handle format_auto,animated

[jameswragg]

Fixes #234

Using animated modifier creates the following modifier key/value which wasn't seen as a truthy value when used in combination with format_auto meaning the format selection wasn't limited to "image/webp", "image/gif":

{
  animated: ''
}

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.83%. Comparing base (f666cb4) to head (29a216d).
Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
src/server.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #235       +/-   ##
===========================================
- Coverage   56.02%   45.83%   -10.20%     
===========================================
  Files          14       13        -1     
  Lines        1203      888      -315     
  Branches       83       83               
===========================================
- Hits          674      407      -267     
+ Misses        526      478       -48     
  Partials        3        3               

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[pi0]

Thanks. We should later properly fix it by making sure modifiers are mapped to boolean. I have made slight change to explicitly include "" case.