fix(ssr): clone extraProps in SSR HOC

[eligao]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• fixes "object is not extensible" in HOC getInitialProps
  

[codecov]

Codecov Report

Merging #4362 into 2.x will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              2.x    #4362   +/-   ##
=======================================
  Coverage   40.80%   40.80%           
=======================================
  Files         284      284           
  Lines        8079     8079           
  Branches     1968     1968           
=======================================
  Hits         3297     3297           
+ Misses       4409     4402    -7     
- Partials      373      380    +7     

Impacted Files	Coverage Δ
packages/umi/src/renderRoutes.js	92.47% <ø> (ø)
packages/umi-plugin-ui/src/bubble/index.jsx	0.00% <0.00%> (ø)
...kages/umi-plugin-ui/src/plugins/dashboard/index.ts	0.00% <0.00%> (ø)
...s/umi-build-dev/src/plugins/commands/block/util.ts	33.11% <0.00%> (ø)

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[eligao]

Brief explanation:

1. here extraProps is passed as a reference, all the way from the props of the root RouteWrapper, and react props are not modifiable
2. in the case when WrappedComponent.getInitialProps returns the provided prevInitialProps: extraProps directly
3. here we would be attempting to add a property to the immutable object, which throws TypeError: can't define property "x": "obj" is not extensible