Accept any token to be a valid request method

- Rename read_until_space() to read_token_until_space()
- Check if the byte is part of the token in read_token_until_space()
- Return HttpMethodError if the token is empty
- Remove the unnecessary is_valid_method() function
Merge pull request #139 from barosl/is_uppercase

std::char::is_uppercase() -> std::char::UnicodeChar::is_uppercase()