Update call.js

If you: throw "a string"; sinon stores it directly in this.exception, and not as an object.

Updated to check to see if this.exception is a string, and if so compare directly with string being checked against.
String Based Threw Fixes

Cleaned up if check per cjohansen's request.
Added test cases per mantoni's request.
Update call.js

Combined last two return statements and switched == to ===
Merge pull request #265 from brandonheyer/patch-1

Update call.js