Fix crash in MIMEScanner::append with std::string.

[SolidWallOfCode]

I think this is a fix for #5587. The problem was Http2Stream wasn't calling the HTTP parser initialization and destruction correctly. http_parser_init needs to be called every initialization after a proxy allocate, not just in the constructor. Converserly, http_parser_clear needs to be called in the destroy method. Otherwise the parser (which contains the MIME scanner) is left in a bad state.

[masaori335]

For the initialization, http_parser_init() finally go to the MIMEScanner::init() and initialize the MIMEScanner::m_line with placement new correctly.

This looks reasonable.