I just want to point out that single-quoted string is considered a "token" according to RFC HTTP/1.1, not "quoted-string". So I'm not sure about all the consequences this change can cause in terms of regression.
Igor, single quotes in "Accept" header is quite common for Android 2.x web clients. Current version of Spring WebMVC denies all requests from that browsers. I think it is too big penalty for RFC violation. Maybe is is better to ignore all parse errors of Accept header and work as if headers is missing in request.
I think this is not yet fixed in 3.1.3. The following commit applied a "fix" in master. However as pointed out above, a single quote signifies a token rather than a quoted string. Some further analysis is needed.