Improve Message to be more correct.

[pyfisch]

• Allow responses to HEAD requests with body size.
• Enforce informational messages without body.

Improved documentation, restructured the Message state machine. Now if a method of the machine is called it will move the FSM to another state and always the same state except if an error occurs. For this reason the new FSM is more understandable. Also it occupies 8 bytes less of storage.

Which messages contain a body in defined in RFC 7230

I have also created a graph of the state machine. It could be included in the documentation once rustdoc gains support for embedding images.

Source code:

digraph Message {
    ResponseStart -> FinalResponseStart [label=response_continue]
    ResponseStart -> Headers [label=response_status]
    FinalResponseStart -> Headers [label=response_status]

    RequestStart -> Headers [label=request_line]

    Headers -> Headers [label=add_header]
    ChunkedHeaders -> ChunkedHeaders [label=add_header]
    FixedHeaders -> FixedHeaders [label=add_header]
    Headers -> FixedHeaders [label=add_length]
    Headers -> ChunkedHeaders [label=add_chunked]

    Headers -> Bodyless [label=done_headers]
    FixedHeaders -> FixedBody [label=done_headers]
    ChunkedHeaders -> ChunkedBody [label=done_headers]

    FixedBody -> FixedBody [label=write_body]
    ChunkedBody -> ChunkedBody [label=write_body]

    Bodyless -> Done [label=done]
    FixedBody -> Done [label=done]
    ChunkedBody -> Done [label=done]

    Done [peripheries=2]
    Done -> Done [label=done]

    {rank = same; RequestStart; ResponseStart;}
    Request_invis [shape=plaintext label=start]
    Request_invis -> RequestStart
    Response_invis [shape=plaintext label=start]
    Response_invis -> ResponseStart

    ResponseStart [tooltip="ResponseStart { version: Version, body: Body, close: bool }"]
    FinalResponseStart [tooltip="FinalResponseStart { version: Version, body: Body, close: bool }"]
    Headers [tooltip="Headers { body: Body, close: bool }"]
    FixedHeaders [tooltip="FixedHeaders { is_head: bool, close: bool, content_length: u64 }"]
    ChunkedHeaders [tooltip="ChunkedHeaders { is_head: bool, close: bool }"]
    Bodyless [tooltip="Bodyless"]
    FixedBody [tooltip="FixedBody { is_head: bool, content_length: u64 }"]
    ChunkedBody [tooltip="ChunkedBody { is_head: bool }"]

}

[tailhook]

Looks great! Thanks.