Some cleanup

Most are minor and small non-functional ("styling") changes. I've left the commits as they are to make things more clear, but I would like to squash it before merge. Every commit has a comment.

- @a51875b6
    Is there is a reason, why in line 389 and following the `$value` gets encoded and decoded over and over again?

- I found a test, that simply didn't worked. I refactored it into 3 separate tests (@c63f6c6), but now 2 of those fails, because `Filter::__construct()` doesn't validate the arguments at all, but exactly this was, what the tests should ensure. I've marked them as skipped for now (I think "incomplete" would have been the better choice :X).


And a question at the end: I've seen many many `preg_*()`-calls. Couldn't some of them get replace by `str_replace()`? Or (because this is the first thing, that comes to my mind) are there any issues with special characters?

Enough for now :smile: