Fix header parsing for values that span more than one call to _on_header_value

[wch]

This fixes #275.

cc @aronatkins

[aronatkins]

Do fields need the same "append" treatment? It's possible to see multiple calls for both field names and values according to the table at https://github.com/nodejs/http-parser/blob/5b76466c6b9063e2c5982423962a16f7319c81f8/README.md#callbacks

[wch]

The fields already had the same append treatment -- the std::copy with the std::back_inserter appends incoming data to _lastHeaderField.

httpuv/src/httprequest.cpp

Line 234 in 7243a57

std::copy(pAt, pAt + length, std::back_inserter(_lastHeaderField));

(_lastHeaderField is now cleared when _last_header_state transitions to FIELD.)

I've also added a unit test for this.