Fix auto_increment support in cast rules.

This fixes #141 again when users are forcing MySQL bigint(20) into
PostgreSQL bigint types so that foreign keys can be installed. To this
effect, as cast rule such as the following is needing:

   cast type bigint when (= 20 precision) to bigint drop typemod

Before this patch, this user provided cast rule would also match against
MySQL types "with extra auto_increment", and it should not.

If you're having the problem that this patch fixes on an older pgloader
that you can't or won't upgrade, consider the following user provided
set of cast rules to achieve the same effect:

   cast type bigint with extra auto_increment to bigserial drop typemod,
        type bigint when (= 20 precision) to bigint drop typemod

src/sources/common/casting-rules.lisp

@@ -47,7 +47,7 @@
 		((:typemod typemod-expr) nil tm-s-p)
 		((:default rule-source-default) nil d-s-p)
 		((:not-null rule-source-not-null) nil n-s-p)
-		((:auto-increment rule-source-auto-increment) nil ai-s-p)
+		((:auto-increment rule-source-auto-increment))
 		&allow-other-keys)
 	rule-source
       (destructuring-bind (&key table-name
@@ -69,7 +69,11 @@
 	     (or (null tm-s-p) (typemod-expr-matches-p typemod-expr typemod))
 	     (or (null d-s-p)  (string= default rule-source-default))
 	     (or (null n-s-p)  (eq not-null rule-source-not-null))
-	     (or (null ai-s-p) (eq auto-increment rule-source-auto-increment)))
+
+             ;; current RULE only matches SOURCE when both have an
+             ;; auto_increment property, or none have it.
+             (or (and auto-increment rule-source-auto-increment)
+                 (and (not auto-increment) (not rule-source-auto-increment))))
 	  (list :using using :target rule-target))))))
 
 (defun make-pgsql-type (source target using)