Do not add content-length if chunk reply.

[bluthen]

We shouldn't add Content-Length header if we are doing a chunked transfer. It is fine on most browser but some, like Safari, have issues with it.

See:
https://stackoverflow.com/questions/3304126/chunked-encoding-and-content-length-header
http://greenbytes.de/tech/webdav/rfc2616.html#rfc.section.4.4 (point 3)

[NathanFrench]

After looking over things, I decided to rework a bunch of stuff related to this.

From rfc7230 section 3.3.2 (https://tools.ietf.org/html/rfc7230#section-3.3.2), there are a lot of things we seem to be missing:

   A server MUST NOT send a Transfer-Encoding header field in any
   response with a status code of 1xx (Informational) or 204 (No
   Content).  A server MUST NOT send a Transfer-Encoding header field in
   any 2xx (Successful) response to a CONNECT request

   A server MUST NOT send a
   response containing Transfer-Encoding unless the corresponding
   request indicates HTTP/1.1 (or later).

   A server MAY send a Content-Length header field in a response to a
   HEAD request (Section 4.3.2 of [RFC7231]); a server MUST NOT send
   Content-Length in such a response unless its field-value equals the
   decimal number of octets that would have been sent in the payload
   body of a response if the same request had used the GET method.

And one that seems to relate the most to this patch

   A sender MUST NOT send a Content-Length header field in any message
   that contains a Transfer-Encoding header field.

If we look at https://github.com/criticalstack/libevhtp/blob/develop/parser.c#L1535 I'd say we could use this information, expose a header type enum, and we don't have to strcmp twice.

Thank you for pointing this out, I have a bit of work ahead of me :)

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