Handle :default => false correctly

[jfirebaugh]

Fixes #143.

I mentioned :default => nil in the issue, but that's harder to support and not something I actually need, so this PR doesn't address it.

[bogdan]

+1 for this

[ndbroadbent]

+1, please consider merging this. Currently, I'm doing :default => 0, and then testing for 0.

[radar]

Thanks for the PR! It's great that it still applies. I was able to work out a solution that worked for nil too, and it's on the default-false-or-nil branch. Once that's green, I'll merge this to master.

[radar]

I've just merged this branch to master.

Thanks!