Rewrite this to be easier to understand and more correct, and make the

Rewrite this to be easier to understand and more correct, and make the

	* libsoup/soup-headers.c (soup_headers_parse): Rewrite this to be
	easier to understand and more correct, and make the "str" param
	const rather than overwriting it during parsing.
	(soup_headers_parse_request, soup_headers_parse_response):
	Likewise, make "str" param const. Fix the doc comment to describe
	the correct constraint on str. Make the parsing slightly more
	lenient as per sections 4.1 and 19.3 of RFC 2616.

	* tests/header-parsing.c: new regression test, for Request-Line,
	Status-Line, and message-header parsing.

	Inspired by #391970 (crash in SoupServer when certain invalid
	requests are received).

svn path=/trunk/; revision=905