collection_size locals in collection partials

This patch adds collectionsize, firstincollection and lastin_collection local assigns to partial collections.

it basicly just sets:

local_assigns[:collection_size] = collection.size
local_assigns[:first_in_collection] = index == 0
local_assigns[:last_in_collection] = index == collection.size-1

in the renderpartialcollection method.

this gives you more flexibility in designing partials that are rendered in collections.

Issues: since the local assigns are just added to partials that are renderd in collections it is not available if a single partial is rendered.