Request::parse() accepts invalid methods.

[maxbailly]

This issue seems to be related to #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