Improve HTTP_RANGE support, Fix #380

 - Added a test to check whether "bytes=" is supported
 - added a simple way to support the "bytes=" prefix by stripping it if present
This should not break backward compatibility.
After reviewing the RFCs, it seems like we can expect the 'bytes=' prefix to always be present (I'm not sure why that wasn't always the case in this implementation!) I think it would be good to require it to be present in 0.3, but for now I think this patch is good as-is.