jackc/tod — pull request #20 by peteryates (2014-12-31 12:16:31+00:00)
Title: return nil unless `other` is comparable

[This](https://github.com/ruby/ruby/commit/d781caaf313b8649948c107bba277e5ad7307314) commit caused the following warnings to appear when comparing objects.

Comparable#== will no more rescue exceptions of #<=> in the next release.
Return nil in #<=> if the comparison is inappropriate or avoid such comparison.

This fix returns nil unless `other` is comparable in the same style as [a similar fix](https://github.com/rdoc/rdoc/commit/23c276de) in rdoc