jion方法的条件支持传入Expression对象

library/think/db/Builder.php

@@ -545,7 +545,9 @@ abstract class Builder
                 list($table, $type, $on) = $item;
                 $condition               = [];
                 foreach ((array) $on as $val) {
-                    if (strpos($val, '=')) {
+                    if ($val instanceof Expression) {
+                        $condition[] = $val->getValue();
+                    } elseif (strpos($val, '=')) {
                         list($val1, $val2) = explode('=', $val, 2);
                         $condition[]       = $this->parseKey($val1, $options) . '=' . $this->parseKey($val2, $options);
                     } else {

library/think/db/Query.php

@@ -924,7 +924,7 @@ class Query
      * @access public
      * @param string|array $table 数据表
      * @param string|array $field 查询字段
-     * @param string|array $on    JOIN条件
+     * @param mixed        $on    JOIN条件
      * @param string       $type  JOIN类型
      * @return $this
      */