`read_headers` should be case-insensitive on keys

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Device and/or platform:

Description of problem:
read_headers is not case-insensitive on keys, as described in the standard.
This can lead to problems, if the client uses different cases for ie. content-length.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers

Expected:
Lowercase all the received headers.

Traceback (if applicable):

Additional info: