fix: animated modifier wasn't correctly being passed to autoDetectFormat

[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[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 56.02%. Comparing base (f666cb4) to head (364d77d). Report is 16 commits behind head on main.

Files	Patch %	Lines
src/server.ts	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #235 +/- ## ======================================= Coverage 56.02% 56.02% ======================================= Files 14 14 Lines 1203 1203 Branches 83 84 +1 ======================================= Hits 674 674 Misses 526 526 Partials 3 3 ```

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