Improve HTTP::Response::Status

- Cover with tests edge cases of String reason Symbolization
- Provide extra map of Symbols to codes and codes to Symbols
- Provide simple way to coerce symbols/codes to Status instance

``` ruby
HTTP::Response::Status[:bad_request] # => 400 Bad Request
HTTP::Response::Status["Not found"]  # => 404 Not Found
HTTP::Response::Status[500]          # => 500 INternal Server Error
```