Fix fallback behavior of ruby-electric-space/return.

Calling this-original-command may fail in some situations like it is
ac-complete that has been triggered when there is nothing left to
complete.

A function ruby-electric-space/return-fallback is introduced that falls
back to a globally bound feature when such an error occurs.
Reword some docstrings.
Introduce a new featrue `ruby-electric-autoindent-on-closing-char`.
v2.1