Fix first_image not resolving correctly when in subdirectories and ogp_image is defined.

[ItayZiv]

Fixes #59 again.
The fix in #61 doesn't work properly when a ogp_image is defined in the config.
Swaps the order of the if's and fixes the tests so it will detect this case.

[TheTripleV]

If the condition on L125-L128 fails, then first_image is not None and ogp_use_first_image == True but it shouldn't be using page_url as the root.

Can you add in a

else:
	first_image = None

on that statement? (Github won't let me suggest there)