Read/Write detection doesnt work correctly for resources from gzopen

[mcfedr]

When opening a file with gzopen it is possible to specify some extra options, compression level, etc

When a Stream is created it detects readable/writable checking the mode with $readWriteHash which doesn't have entries for all possible modes, eg 'rb9' or wb2.

This results in the Stream being incorrectly marked as neither readable or writable.

I would be happy to make a pull request, but thinking about the best solution. It would be possible to add all the variations to $readWriteHash, but maybe some string manipulation is better option.

[Tobion]

I'd be happy to see a PR with a test. This is related to #183. So if you come up with an easy solution usingg string manipulation, that's fine with me as well.