Clarify that requests with Content-Length: 0 do not require a Content…

[JoostK]

…-Type header

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This PR fixes issue #1508.

[randomstuff]

For requests with Content-Length: 0 no Content-type header is expected.

Is there an ambiguity here whether this is 1) "we might not have a Content-type in some cases" or 2) "you must not use a Content-Type"? I would claim we want (1).

[jmanico]

I think "we might not have a Content-type in some cases" is the better direction here. There are some API's that will require it even with empty content.

[JoostK]

For requests with Content-Length: 0 no Content-type header is expected.

Is there an ambiguity here whether this is 1) "we might not have a Content-type in some cases" or 2) "you must not use a Content-Type"? I would claim we want (1).

I was trying to avoid this but the current wording does indeed leave this ambiguity. I mentioned some potential alternatives in #1508 (comment).