Fix mismatching variable names when using an underscore

The ERBTracker template digest helper class was using a regex to match
render calls and it was incorrectly not matching against variables with
underscores in the name. This caused it to use the wrong regex match data
to populate the template dependency. Because underscore is a valid
character for a variable, this fixes the ERBTracker to match it properly.
Merge pull request #10841 from latortuga/fix-underscore-dependency-tracking

Fix mismatching variable names when using an underscore