Trim provided extension key

[georgringer]

A colleague entered as extension key content_elements with a whitespace at the end which was unnoticed but lead to have a directory name with the whitespace in it.

As there was already an extension content_elements without the whitespace that lead to have 2 extension directories which looked pretty same.

I know that it is a mistake of the user but providing a trim or maybe even not allowing whitespace to enter would reduce the risk of bugs