修复 beforeSave 没有正确返回对象的 bug

[juvenn]

用户定义的函数不需要返回结果，应该由中间件来负责返回在函数中被修改的对象。

[codecov-io]

Codecov Report

Merging #149 into master will increase coverage by 0.03%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   67.66%   67.69%   +0.03%     
==========================================
  Files          33       33              
  Lines        2134     2133       -1     
==========================================
  Hits         1444     1444              
+ Misses        690      689       -1

Impacted Files	Coverage Δ
src/LeanCloud/Client.php	85.15% <ø> (ø)	:arrow_up:
src/LeanCloud/Engine/LeanEngine.php	0% <0%> (ø)	:arrow_up:

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[juvenn]

Approved