Fix Bug 1038392 - Image plugin test urls are no longer reachable

https://bugzilla.mozilla.org/show_bug.cgi?id=1038392
Didn't think about changing it to an else if. The reasoning behind the change is mostly clarity and the readability. Plus jshint complains about no assignment, plus performance wise it is much slower vs just using an if.

Overall the code should be clear and easy to read, no need to "minify" it, that is the job of the closure compiler.