Implement support for SQLite current_date default value.

The spelling in SQLite for the default value is "current_date", instruct
pgloader about that. This commit also adds a test case in our sqlite.db
unit tests database.

Fixes #607.

src/sources/sqlite/sqlite-cast-rules.lisp

@@ -100,6 +100,8 @@
                         (or (uiop:string-prefix-p "CURRENT_TIMESTAMP" default)
                             (string= "CURRENT TIMESTAMP" default)))
                    :current-timestamp)
+                  ((and (stringp default) (string-equal "current_date" default))
+                   :current-date)
                   (t (column-default pgcol))))
 
       pgcol)))