Fix to properly handle a Range request with a last byte pos beyond the content length (CVP2RI-356)

HTTPByteSeekRequest was reporting an error when the range end is beyond
 the content length. Per DLNA 7.5.4.3.2.22.6 (and IETF RFC 2616 14.35.1),
 the "last byte pos" value may be >= the size of the content (when known).
 So changed HTTPByteSeek to treat a request with a range end beyond the
 content size as a range from the requested start to end of content.

This is consistent with how SoupMessage.set_range() operates.

Tested with various Range requests. Previous forms that returned an error
 due to an out-of-range now return data to the end of the resource.
Merge branch 'cablelabs/master' of github.com:cablelabs/rygel into cablelabs/master