Bug fixes/enhancements from testing with odid MediaServer/MediaEngine

Separated check for GET/HEAD from getcontentFeatures header check in
 HTTPGet.handle () while trying to debug failing GETs. This qualifies
 as "cleanup". But think it's worth retaining.

Testing with odid flushed out some assumptions that all MediaItems
 were MediaFileItems.

Fixed an assumption in the HTTPGet.find_item() implementation. The
 check for a request on an placeholder MediaFileItem was not checking
 that the object was a MediaFileItem. Added the check.

HTTPRequest.find_item() has an over-complicated check to flag a
 non-playlist URI request that wasn't for a MediaContainer or a playlist
 URI request that referred to a MediaItem. This was incorrectly changed
 to refer to a MediaFileItem. Changed the check back to MediaItem
 (and resisted cleaning this up and adding to the diff).