Resolve MiniTest deprecation messages and warnings
Fix a bug where passing nil to sanitize-rails helpers would raise exceptions

The exception raising behavior is also out of the norm for the usual API
for rails helpers: most helpers, when provided a nil input,
simply return nil or a blank string.

This commit changes the
`Sanitize::Rails::Engine#clean` and `Sanitize::Rails::Engine#clean!`
methods such that they return blank strings when provided nil inputs.
Merge pull request #8 from damien/handle-nil-inputs

Handle nil inputs in Sanitize::Rails::Engine#clean and #clean!