Change AttributeKey matching to singleton matching

Removes the ClassTag and makes it impossible to generate imposter keys, forcing a one to (one or none) correspondance between key and entry whereas it had been possible to generate "imposter" keys by using the same name and type.
Update comments to reflect the new reality of key equality.
Merge pull request #84 from http4s/topic/SingletonAttributeKeys

Change AttributeKey matching to singleton matching