Revert "Use IO.autoclose on 1.9+ for fdopen"

This reverts commit 8c03eb0daa997273882b27a27a4003c2097d68e1.
Revert "Explicitly close file objects on fdopen"

This reverts commit c02c78b3899fcf769084a88777c63de0fcebb48d.
Fixed File#open, #reopen properly.
Fixed spec description.
Fixed style of Enumerable#group_by specs.