unsetAll(), nullifyEmptyStrings

unsetAll is a convenience method that will remove all elements specified by the `$keys` array.

nullifyWhiteSpace will convert all strings containing only whitespace to null. Usefull for db inserts when you want to insert null and not an empty string.
Your documentation says "empty strings", but this does `trim` first.  I think the `trim` call should either be optional (triggered by a flag) or be done first (say, using the `dominionenterprises/filter` package).