Add a couple more request variables

[digitalresistor]

This copies the list from #110

• content_language
• content_range
• content_md5
• expect
• expires
• from_email
• via
• warning

[luhn]

Looking to get my toes wet in webob, so I was looking over these.

Does webob aim for completeness (relative to the RFC) or usefulness? HTTP usage is pretty vanilla these days, I don't see many of these headers being used, but if we're looking to fully implement the RFC we need them.