Split content parsing and level validating

[antiphoton]

The old function validate_accept_header has been doing two tasks

1. Parse the q value of SXG content type
2. Validate that the q value meets the requirements.

This PR decouples the two tasks.

• Diff in production code

  - assert!(validate_accept_header(s, required).is_ok());
  + let actual = parse_accept_value(s);
  + assert!(actual >= required);

• Diff in testing code

  - assert!(validate_accept_header(s, PrefersSxg).is_ok());
  - assert!(validate_accept_header(s, AcceptsSxg).is_ok());
  + assert_eq!(parse_accept_level(s), PrefersSxg);

  - assert!(validate_accept_header(s, PrefersSxg).is_err());
  - assert!(validate_accept_header(s, AcceptsSxg).is_ok());
  + assert_eq!(parse_accept_level(s), AcceptsSxg);