Handle missing key when PlayQueue passed to playMedia

[jjlawren]

If a PlayQueue object is passed to PlexClient.playMedia(), the generated key argument used in the resulting call is always None. This uses the currently selected item from the PlayQueue in that case.

[jjlawren]

An alternative that's less explicit but far simpler: jjlawren@bb7b959

[jjlawren]

The key value has been sending None to clients for years when a premade playqueue is passed in, but most seem to handle it gracefully and play anyway. Others like the 3rd party Samsung TV "Orca" client do not and throw up an error.