Fix warnings on < 5.8.9

Explicitly assigning undef to a %SIG handler is not supported on < 5.8.9. This results in warnings, and leads among other things to a failure in the DBIC test suite.

A plain local() call is sufficient to "unset" the handler on all perls.
Merge pull request #4 from ribasushi/patch-1

Fix warnings on < 5.8.9