fix(callback): callback was called twice when lib was already loaded

[gonzaloruizdevilla]

When $script was used with lists with shared scripts, the inside callback() was called twice for previously loaded libraries, and this caused the done() to be called before the scripts were loaded.
Side bonus: 21 bytes less in minified library

[gonzaloruizdevilla]

@ded Do you think this PR will take long to be merged?
It resolves a very importante bug in the library (blocker in my projects).

I didn't explain the bug clearly.

Lets say you do:

$script(['foo.js', 'bar.js'], doSomething)

and afterwards you do

$script(['foo.js', 'bar.js', 'baz.js'], doSomethingElse)

doSomethingElse may be called before 'baz.js' has been loaded and executed.

Thank you!

[ded]

i see now. thanks for the test. let me make sure it didn't break any other edge cases