RFR: 8263506: Make sun.net.httpserver.UnmodifiableHeaders unmodifiable [v3]

[Julia Boes]

On Thu, 1 Apr 2021 14:08:47 GMT, Michael McMahon <michaelm at openjdk.org> wrote:

>> Julia Boes has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   move constructor call in ExchangeImpl and fix indentation
>
> LGTM

I included a null check in the constructor to account for null values of `List<String>`, which `Headers` allows. Fixed the indentation.

Testing: Tier 1-3 all clear.

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PR: https://git.openjdk.java.net/jdk/pull/3032