Revert "Fix Ruby 1.8.5-incompatible code in Transaction#eval_generate"

This reverts commit 737c2f66476b2379f2546150856ea1cd7fdde08a.

The commit above was implemented to maintain compatibility with ruby
1.8.5, but resulted in a severe performance penalty ~500 times slower.
Since Puppet 3 is not compatible with ruby 1.8.5, this commit restores
the original version which is much faster.