Un-underscore header names provided by Rack

[kainosnoema]

This fixes two problems:

• This will now properly convert underscored header names to their dashed equivalent. Custom headers like Content-MD5 turns into HTTP_CONTENT_MD5 in Rack, so converting back from CONTENT_MD5 to CONTENT-MD5 is required.
• Sometimes Rack's header values are nil. The one I've encountered so far using Rails in development is Origin. This will only set the header if the value is non-nil.

[emilianogfcm]

You can do a release for this?