Support detecting request body

[clue]

Proper detection of request message bodies requires:

• Content-Length
• Chunked transfer encoding (Support chunked transfer encoding #96)
• Empty bodies for certain requests

See https://tools.ietf.org/html/rfc7230#section-3.3

This means that we should take these into account and only emit data events for what is actually part of the given request and then emit an end and close event.

This is also a prerequisite for #39.

[WyriHaximus]

While seeing all the prerequisites are met, doesn't this make more sense to come in 0.8?

[clue]

This PR only handles detecting the message body, i.e. emitting streaming data, end, error and close events. Afaict this is not handled for all cases yet (requests with implicit message lengths). Once everything is completed, this should be released as v0.6.0.

This ticket does not handle parsing this message body, this is left up for #105. Parsing the body relies on detecting its length first (this ticket) and likely also on adding a proper interface first (#28), so this is currently planned for the v0.8.0 release.

[clue]

Closed via #116, #129, #130, #136 and #137. Example added via #139.