Bugfix/filter parser fails to parse blackets

I forgot I sent a pull-request #72 to the upstream repository. the upstream has been evolving since I sent the pull request. 
So I've merged the head of the branch master and rewrite a test code. 

@jch Thank you for informing me. 
@jch 

> What does it mean, and how is it used? 

Do you point this `cn=[{something}]` ? if so, this filter does not make sense as a filter. 
As I read [the article](http://www.rlmueller.net/CharactersEscaped.htm), I came to think FilterParser should parse a given string including `[]{}` as a filter.