Split 'write_head' into 'write_status_line' and 'write_headers'

[arteymix]

Some protocols like 'HTTP/2.0' use different channels for sending headers and body resulting in the possibility of having parts of the body emitted while not all headers are sent. This is while splitting Response.write_head is very convenient.

Automatically calling a writer should be implementation-specific, so we don't write headers when the body is first accessed.

Response.head_written is replaced by a flag covering the following conditions:

• status line written
• headers written

It's not possible to reliably know when the body has been written.

It's handy to have a final status line but not yet headers, especially if one wants to generate headers based on a specific status.

res.wrote_status_line.connect (() => {
    // status line is final
    if (res.is_cacheable ()) {
        res.headers.append ("Cache-Control", "public");
    }
});

There's some tests to fix and probably add cover new edge cases.

[codecov-io]

Current coverage is 64.88%

Merging #186 into master will decrease coverage by 0.70%

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- Misses          303        318    +15   
  Partials         37         37          

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