Add new type DateString to fromColumnValue

To sync with loopback-datasource-juggler #1356
which introduces new Type: DateString

Author: bbito

lib/mysql.js

@@ -352,6 +352,9 @@ MySQL.prototype.toColumnValue = function(prop, val) {
     }
     return val;
   }
+  if (prop.type.name === 'DateString') {
+    return val.when;
+  }
   if (prop.type === Boolean) {
     return !!val;
   }
@@ -405,11 +408,11 @@ MySQL.prototype.fromColumnValue = function(prop, val) {
         val = String(val);
         break;
       case 'Date':
-
+      case 'DateString':
         // MySQL allows, unless NO_ZERO_DATE is set, dummy date/time entries
         // new Date() will return Invalid Date for those, so we need to handle
         // those separate.
-        if (val == '0000-00-00 00:00:00') {
+        if (!val || val == '0000-00-00 00:00:00' || val == '0000-00-00') {
           val = null;
         }
         break;