make parse_date only work on Strings

If the argument is not a String, the argument is returned.
Without this check, it was not possible any more to assign i.e.
Time.now to an attribute which has been calendar_date_attribute'ed.
With this check, only Strings are parsed, the rest is passed though.

lib/calendar_date_select/calendar_date_select.rb

@@ -78,6 +78,7 @@ def format_date(date)
 
     # parses a date string using the specified format
     def parse_date(date)
+      return date unless String===date
       if has_time?(date)
         hash = ::Date._strptime(date, date_format_string(true))
         return nil if hash.nil?