Fixes an issue when you pass an url directly.

This happens for example when you fetch og.likes with
a next URL which is already with all the necessary fields in it.
With this fix you're able to pass a fully constructed URL (so,
in this case, it's merely a wrapper around `request` but it
helps when you can use the same API).
Add documentation to README about calling directly an URL

NB: might not be the best wording
Merge pull request #34 from mweibel/fix-passing-url

Fixes an issue when you pass an url directly.