Cast x-spread expression to string

[KevinBatdorf]

This breaks because expressions are functions in x-spread, and when the output is defined, it attempts to run a String operation on it.

 if (output === undefined && expression.match(/\./)) {
     output = ''
 }

...which causes an error. This fix will just cast the function there to a string so it won't error out.

Fixes #957

[KevinBatdorf]

Also snuck a little note on x-ref but can remove that if you want

[calebporzio]

Thanks Kevin!