If called with a hash that has a `default` / `default_proc`
configured, `Devise::ParameterFilter` can add in missing keys
it was due to attempt to sanitise the values for.

This patch prevents this from happening, whilst also clarifying
the filtering intent of `ParamaterFilter`.

(This can also occur if NilClass has been augmented with definitions
for `strip` or `downcase`.)

Fixes #3431.