fix: case-insensitive FormField headers

[jgranstrom]

Some clients send FormField headers in lower case. However we seem to require them to be capitalized. I can't find a clear spec for headers in this context, but from headers in general we can probably expect it to vary. One example where we're incompatible is react-native.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (1408f9c) 64.28% compared to head (26d53e1) 64.80%.
Report is 3 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2460      +/-   ##
==========================================
+ Coverage   64.28%   64.80%   +0.51%     
==========================================
  Files         135      135              
  Lines        7101     7129      +28     
  Branches     1211     1185      -26     
==========================================
+ Hits         4565     4620      +55     
+ Misses       2536     2509      -27     

Files	Coverage Δ
zio-http/src/main/scala/zio/http/MediaType.scala	96.55% <100.00%> (+0.12%)	⬆️
.../src/main/scala/zio/http/codec/RichTextCodec.scala	88.58% <100.00%> (+4.52%)	⬆️
zio-http/src/main/scala/zio/http/FormField.scala	88.00% <75.00%> (ø)
zio-http/src/main/scala/zio/http/Header.scala	63.46% <92.72%> (+2.09%)	⬆️

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