Collection [] accessor not always working.

[maxdupenois]

When attempting to use the [] accessor on collection it will not always return the correct resource.
For example, when accessing lists:

api.account.lists[LIST_ID] #=> returns an empty resource

api.account.lists.find_by_id(LIST_ID) #=> returns the correct list

I suspect this is because the call to [] gives:

    def [](id)
      @entries[id] ||= fetch_entry(id)
    end
...
    def fetch_entry(id)
      @klass.new(client, get(path).merge(:parent => self))
    end

Where fetch_entry does not make use of the passed id, resulting in an empty resource.

[patrickmclaren]

When a collection is initialized, ws.size entries are fetched. Trying to access an entry beyond the first page of results will hit this bug.