Fix contrib/btree_gist to handle collations properly.

Fix contrib/btree_gist to handle collations properly.

Make use of the collation attached to the index column, instead of
hard-wiring DEFAULT_COLLATION_OID.  (Note: in theory this could require
reindexing btree_gist indexes on textual columns, but I rather doubt anyone
has one with a non-default declared collation as yet.)