Bugs in `Authorization: Bearer …` header parsing

Start at the headers::authorization::Bearer implementation and run the test_invalid_auth_header example from the issue. Check parsing of malformed values, restricted token characters, extra spaces, and case-insensitive auth schemes; done means invalid headers fail and valid tokens are returned without surrounding whitespace.

The following test exposes a few issues in headers::authorization::Bearer:

#[cfg(test)]
mod tests {
  #[test]
  fn test_invalid_auth_header() {
    // Malformed, should not parse.
    let value = http::HeaderValue::from_str("Bearer    foo, Bearer bar").unwrap();
    use headers::authorization::Credentials;
    let decoded = headers::authorization::Bearer::decode(&value);
    println!("decoded = {:?}", decoded);
    println!("if decoded succeeded, the token was: {:?}", decoded.as_ref().map(|b| b.token()));
    // The header is malformed, so parsing should fail:
    assert!(decoded.is_none());
  }
}

Basically, Bearer's implementation will strip the leading string "Bearer " from the value, and return the remainder for as a token. This isn't valid according to the grammar for the Bearer authn scheme. (The grammar of the token itself is restricted to a certain set of characters, for example.) In fact, the code only checks that the header starts with Bearer in debug mode (as it is done w/ a debug_assert!), so in a release build, any value will parse, including, e.g., Basic <a basic headers' credentials>.

Even where the header is valid, and should parse, the .token() determines the token by simply removing "Bearer ".len() characters from the front of the header value, which itself is not correct: Bearer foobar (n.b., the double space; this is permitted by the grammar for the header, but is not part of the token) parses as it should, but .token() returns " foobar".

Last, the debug-mode-only debug_assert! asserts that the value starts with Bearer — but auth schemes in HTTP are case insensitive, so this too will reject valid inputs. (Note: the Basic auth-scheme parser shares its own version of this issue, too.)