修复 find => force => save 这个流程下update_time 不更新的问题

[ichynul]

$user = User::find(1);
$res = $user->force()->save(['name' => '小明' . time()]);
//结果：表中`update_time`不更新。

原因分析，find查询出表中的所有字段，包含update_time。
当调用->force()后，更新的时候不检查是否需要更新并unset()未改变字段。
把查询出来的update_time值原样写回去。
在autoDateTime方法中，判断当保存的数据中update_time字段空才会把它设置为当前时间

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.98%. Comparing base (899093b) to head (3419730).

Additional details and impacted files

@@            Coverage Diff            @@
##                4.0     #852   +/-   ##
=========================================
  Coverage     51.98%   51.98%           
- Complexity     3530     3531    +1     
=========================================
  Files            74       74           
  Lines          8314     8314           
=========================================
  Hits           4322     4322           
  Misses         3992     3992           

Flag	Coverage Δ
unittests	51.98% <100.00%> (ø)

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