add parse_http_date.

Signed-off-by: Jack <jack.wgm@gmail.com>
Merge branch 'master' into cookie

Conflicts:
	include/avhttp/detail/parsers.hpp

Signed-off-by: Jack <jack.wgm@gmail.com>
improve alloc object for shared_ptr.

Signed-off-by: Jack <jack.wgm@gmail.com>