[fix] parsing content-type header for actions

[repsac-by]

Fixes #7187

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[elliott-with-the-longest-name-on-github]

Did you see the note from @Conduitry about making sure we're doing this for CSRF? I haven't looked yet to see if any changes are necessary -- just want to leave this not as a "TODO".

[repsac-by]

I saw this note, but I don't see how CSRF can be related to incorrect header parsing.

[dummdidumm]

The joys of coding with the VS Code web extension without any intellisense 😄