Write unit tests for untested utilities

[mstuart]

There are a lot of useful functions in cross-domain-utils, but they aren't all tested! Write some unit tests for untested utilities.

To find what's missing, run npm test, then open coverage/index.html to see what files are missing tests.

[Abdul-Sen]

Hey, I'm trying to get some JS testing experince and this looks like a good opportunity. Please assign me this task and I'll take a look at what useful tests I can add.

[bluepnume]

Let's keep this issue open? There are many utilities in cross-domain-utils which would benefit from unit tests, if any others are looking for a good PR idea for hacktoberfest.

[pushpinder107]

A lot of functions are left untested because they call other (tested) functions internally. It would be quite easy to add more tests if Sinon is added as a dependency.