mod_oembed: Fix setting media title when programmatically importing an oembed media item.

The title used to remain empty when a call like this was made:

    m_rsc:insert([{category, media}, {oembed_url, "http://www.youtube.com/watch?..."}], Context).

This is now fixed, the title is set when a new OEmbed URL is used and
the rsc does not yet have a title.