bug # 20681 delete() function change aproach.

Instead of rejecting elements from the original list, we use
collection = arguments[0].dup .
then latter we could continue to use delete and gsub! on collection
without impact on original argument.

this is a better solution than the previous one, and works on ruby
1.8.7, 1.9.3 and 2.0.0.

The previous solution does not work on ruby 1.8.7.