Fix to share proxy option between proxy settings when the proxy option is a same object

[koba04]

What kind of change does this PR introduce?

a bugfix

Did you add or update the examples/?

No

Summary

Currently, webpack-dev-server mutates the proxy options, which is a cause of bug when the proxy options share a same option object.

For an instance, the following option creates different results after this PR.

• A proxy option

const proxyTarget = {
    target: "http://localhost:9000"
};
proxy: {
    "/proxy1": proxyTarget,
    "/proxy2": proxyTarget,
}

• Before this PR

[ { target: 'http://localhost:9000',
    context: '/proxy2',
    logLevel: 'warn' },
  { target: 'http://localhost:9000',
    context: '/proxy2',
    logLevel: 'warn' } ]

• After this PR

[ { target: 'http://localhost:9000',
    context: '/proxy1',
    logLevel: 'warn' },
  { target: 'http://localhost:9000',
    context: '/proxy2',
    logLevel: 'warn' } ]

I think it should not override the context value.

Does this PR introduce a breaking change?

Maybe not. it is a bug fix.

Other information

[codecov]

Codecov Report

Merging #836 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #836   +/-   ##
=======================================
  Coverage   72.72%   72.72%           
=======================================
  Files           4        4           
  Lines         407      407           
  Branches      119      119           
=======================================
  Hits          296      296           
  Misses        111      111

Impacted Files	Coverage Δ
lib/Server.js	82.65% <100%> (ø)	✅

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[SpaceK33z]

Thanks again!!

[koba04]

Thank you!