Prevent exception if eval called with non-string argument in bab-defuser

[alhimik45]

From MDN:

If the argument of eval() is not a string, eval() returns the argument unchanged.

But eval wrapper in bab-defuser violates this contract: site's scripts get exceptions from check function if pass numbers or functions to eval:

Uncaught TypeError: s.indexOf is not a function
    at check (<anonymous>:23:57)

This pull request adds cheking of argument type.

[mapx-]

@gorhill

[gorhill]

Thanks for catching this.

Could you just change the double-quotes to single-quotes to be consistent with the rest of the file?

[alhimik45]

done