Duplicate georss namespace

[cyberhobo]

This is a long standing problem that was not really resolved in #31, #108, and #399. The root of the problem is that WordPress uses an action (rss2_ns and atom_ns) where a filter would be required for plugins to coordinate output to prevent duplicates.

We could try to buffer the output in an early priority hook and check for the georss namespace in a later priority hook before adding it. Other plugins could feasibly do the same and still get a consistent output.

[cyberhobo]

The committed fix was incomplete, corrected in 1449a15.