fails:Enumerable#group_by returns a hash with values grouped according to the block
fails:Enumerable#group_by returns an empty hash for empty enumerables
fails:Enumerable#group_by returns a hash without default_proc
fails:Enumerable#group_by returns an Enumerator if called without a block
fails:Enumerable#group_by gathers whole arrays as elements when each yields multiple
fails:Enumerable#group_by returns a tainted hash if self is tainted
fails:Enumerable#group_by returns an untrusted hash if self is untrusted
