Patch: Array#to_xml

Array#to_xml always ignores the supplied block if the Array is empty.

There is a ticket with a patch to always yield to the block:
https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2241

I added a patch that won't break the current API. It allows passing in
an ":always_yield => true" option to yield to the block even when
empty. Tests included.

Either patch would make me happy.