fix: table crashes if expandable set without scroll

[Barrywww]

采用了Junlin-Tommy在原PR提出的修改方案，减少对代码逻辑的改动。补充了测试用例。

修复了issue#755第一点提及的问题。

antd的官方文档中并未提及expandable.fixed 和 scroll 两属性需要同时设置，因此将scroll属性置空不应导致整个table crash。采用了默认的auto方式设置水平方向的scroll。

[vercel]

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[codecov]

Codecov Report

Merging #765 (5f178ee) into master (925ff71) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #765   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          35       35           
  Lines         978      978           
  Branches      279      279           
=======================================
  Hits          972      972           
  Misses          6        6           

Impacted Files	Coverage Δ
src/Table.tsx	99.17% <100.00%> (ø)

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[Barrywww]

@afc163 多谢指正！如果3个commit比较乱的话... 需要我rebase成1个force-push上来吗？

[afc163]

不需要，合并了会 squash 掉