Remove instance variable instead of setting it nil
Remove instance variable instead of setting it nil

Since nil is a meaningful value for some instance variables (e.g.
`@normalized_fragment`), I needed to make a distinction between values that are
nil and undefined. I believe this implementation better reflects the comment
"Reset dependent values". I also fixed a typo in that comment.

davidbegin/addressable,Hacker0x01/addressable,shaicoleman/addressable,EiNSTeiN-/addressable,sporkmonger/addressable,sferik/addressable,iainbeeston/addressable