media:group must have multiple media:content children

From the specification [emphasis added]:

<media:group> is a sub-element of <item>. It allows grouping of <media:content> elements that are effectively the same content, yet different representations.  For instance: the same song recorded in both the WAV and MP3 format. It's an optional element that must only be used for this purpose.

Omit the enclosing <media:group> element in cases where there is only one representation.