Currently MockLink hides undefined values in its error message. This
means that debugging a mismatched result was difficult and required
stepping through the code.

There are a couple of possible solutions:

* update the error message to include the undefined values
* modify the `isEquals` comparison of variables to check equality of the
  stringified value

This chooses the latter, since the 99% path for receiving values from a
remote source requires serializing variables before using them.