`Request::parse()` accepts invalid methods.

This issue seems to be related to https://github.com/seanmonstar/httparse/issues/161.

Consider this example :

#[test]
fn invalid_methods() {
    let src = "\"some,unvalid%GET{}:; / HTTP/1.1\r\nHost: 127.0.0.1:80\r\n\r\n";

    let mut headers = [httparse::EMPTY_HEADER; 32];
    let mut req = httparse::Request::new(&mut headers);

    let status = req.parse(src.as_bytes()).unwrap();
    assert!(status.is_complete());
    assert_eq!(req.method, Some("\"some,unvalid%GET{}:;"))
}

From the MDN and my understanding of the RFC, methods MUST be defined by the following grammar:

tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
        "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA

Start at the httparse::Request::parse entry point and reproduce the inline invalid_methods test from the issue. Compare method parsing with the cited RFC tchar grammar. Done means the example no longer accepts characters outside that grammar, with a regression test covering the reported input.