Merge pull request #278 from sanyc/patch-4

修正pgsql驱动的json字段支持参数错误

library/think/db/builder/Pgsql.php

@@ -50,7 +50,7 @@ class Pgsql extends Builder
         $key = trim($key);
         if (strpos($key, '$.') && false === strpos($key, '(')) {
             // JSON字段支持
-            list($field, $name) = explode($key, '$.');
+            list($field, $name) = explode('$.', $key);
             $key                = $field . '->>\'' . $name . '\'';
         }
         return $key;