Fix Spin style when indicator is Icon

[afc163]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Branch merge
• Other (about what?)

👻 What's the background?

close #15693

💡 Solution

Add classNames to i inside Spin.

📝 Changelog description

Describe changes from userside, and list all potential break changes or other risks.

1. English description

• 🐛 Fix Spin style when indicator is Icon. Spin组件的indicator是否只支持Icon组件 #15693

2. Chinese description (optional)

• 🐛 修复 Spin indicator 为 Icon 时的样式。 Spin组件的indicator是否只支持Icon组件 #15693

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

[buildsize]

File name	Previous Size	New Size	Change
package-lock.json	[new file]	799.01 KB

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[codecov]

Codecov Report

Merging #15712 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #15712   +/-   ##
=======================================
  Coverage   95.06%   95.06%           
=======================================
  Files         250      250           
  Lines        6643     6643           
  Branches     1907     1907           
=======================================
  Hits         6315     6315           
  Misses        327      327           
  Partials        1        1

Impacted Files	Coverage Δ
components/spin/index.tsx	94.73% <ø> (ø)	⬆️

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