Cast all values to string before using as an id

The google analytics API can return values which aren't strings, for example
ga:date comes out of the API as a datetime, which was causing this to exit
with an error in the string join.

I'd appreciate a close review because what I did appears to work
but I'm not sure if it is the right thing to do in all cases
we can imagine, and I'm not very familiar with the code.