Firebase-document gives "wrong url" if there is an _ in the fb key

Example:

<firebase-document
 location="https://myfb.firebaseio.com/shows/-KDiuasghoihf923vfk"
 data="{{show}}"></firebase-document>

works but: 

<firebase-document
 location="https://myfb.firebaseio.com/shows/-KDiuasgh_oihf923vfk"
 data="{{show}}"></firebase-document>

does not, as there is a _ underscore in the firebase key - this generates a "Wrong Url" error.
