Fix quoting of default values for MariaDB 10 support.

The default values quoting changed in MariaDB 10, and we need to adjust in
pgloader: extra '' chars could defeat the default matching logic:

  "'0000-00-00'" is different from "0000-00-00"

src/sources/mysql/mysql-schema.lisp

@@ -70,7 +70,7 @@
          (when default
            (babel:string-to-octets default)))
 
-        (t default)))
+        (t (ensure-unquoted default #\'))))
 
 (defun list-all-columns (schema
                          &key

src/utils/quoting.lisp

@@ -44,11 +44,15 @@
 
 (defun quoted-p (s &optional (quote-char #\"))
   "Return true if s is a double-quoted string"
-  (and (eq (char s 0) quote-char)
-       (eq (char s (- (length s) 1)) quote-char)))
+  (or (null s)
+      (when (< 1 (length s))
+        (and (eq (char s 0) quote-char)
+             (eq (char s (- (length s) 1)) quote-char)))))
 
-(defun ensure-unquoted (identifier)
-  (cond ((quoted-p identifier)
+(defun ensure-unquoted (identifier &optional (quote-char #\"))
+  (cond ((null identifier) nil)
+        ((< (length identifier) 2) identifier)
+        ((quoted-p identifier quote-char)
          ;; when the table name comes from the user (e.g. in the
          ;; load file) then we might have to unquote it: the
          ;; PostgreSQL catalogs does not store object names in

test/mysql/my.sql

@@ -1,5 +1,18 @@
 create table `empty`(id integer auto_increment primary key);
 
+CREATE TABLE `races` (
+  `raceId` int(11) NOT NULL AUTO_INCREMENT,
+  `year` int(11) NOT NULL DEFAULT 0,
+  `round` int(11) NOT NULL DEFAULT 0,
+  `circuitId` int(11) NOT NULL DEFAULT 0,
+  `name` varchar(255) NOT NULL DEFAULT '',
+  `date` date NOT NULL DEFAULT '0000-00-00',
+  `time` time DEFAULT NULL,
+  `url` varchar(255) DEFAULT NULL,
+  PRIMARY KEY (`raceId`),
+  UNIQUE KEY `url` (`url`)
+) ENGINE=MyISAM AUTO_INCREMENT=989 DEFAULT CHARSET=utf8;
+
 CREATE TABLE `utilisateurs__Yvelines2013-06-28` (
   `statut` enum('administrateur','odis','pilote','bureau','relais','stagiaire','membre','participant','contact') COLLATE utf8_unicode_ci NOT NULL,
   `anciennete` year(4) DEFAULT NULL,