fix: ignore 'content-type' header capitalization (#1463)

[thislooksfun]

Fixes #1463

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[thislooksfun]

Um... no idea why the tests all failed now. I'm pretty sure that's not my fault.

EDIT: seeing as #1601 also failed in the same way I’m 99% sure it’s a runner failure, and thus not due to this pr.

[Rich-Harris]

Thank you. I made a small change — the headers are already getting lowercased, there's no need to do it again. We can just pass the lowercased headers through to parse_body babe515

[Rich-Harris]

Thank you. I made a small change — the headers are already getting lowercased, there's no need to do it again. We can just pass the lowercased headers through to parse_body babe515

[thislooksfun]

Yeah I thought of that but it's way harder to add a test that way. Ah well, balancing these things is hard. Either way I'm glad it's fixed!