Fixed parsing of absolute URIs with empty path (ticket #2079).

Fixed parsing of absolute URIs with empty path (ticket #2079).

When the request line contains request-target in the absolute-URI form,
it can contain path-empty instead of a single slash (see RFC 7230, RFC 3986).
Previously, the ngx_http_parse_request_line() function only accepted empty
path when there was no query string.

With this change, non-empty query is also correctly handled.  That is,
request line "GET http://example.com?foo HTTP/1.1" is accepted and results
in $uri "/" and $args "foo".

Note that $request_uri remains "?foo", similarly to how spaces in URIs
are handled.  Providing "/?foo", similarly to how "/" is provided for
"GET http://example.com HTTP/1.1", requires allocation.