Raising KeyError if any items in representation set fail -- Fixes #14

  * The original code was allowing non-existant link rels to return
    true in `HalClient::RepresentationSet#has_related?`

  * This was causing a side effect bug when new HalClient::Collection
    instances were trying to be created
Merge pull request #15 from alakra/14_fix_hal_collection_initialization_with_empty_representation_set

Raising KeyError if any items in representation set fail -- Fixes #14