RFR: 8333590: UnmodifiableHeaders.toString() returns a value that represents empty headers [v2]

[Jaikiran Pai]

> Can I please get a review of this change which proposes to fix the issue noted in https://bugs.openjdk.org/browse/JDK-8333590?
> 
> As noted in that issue the `sun.net.httpserver.UnmodifiableHeaders`, an internal class, within the `jdk.httpserver` module doesn't override the `toString()` method. `UnmodifiableHeaders` is of type `com.sun.net.httpserver.Headers` which is a public API in the `jdk.httpserver` module.
> 
> When applications call `Headers.toString()`, if the instance is of type `UnmodifiableHeaders`, then the returned `String` representation shows empty headers, even when it isn't empty. The commit in this PR fixes the issue by implementing `toString()` in the `UnmodifiableHeaders` to return the correct representation of the headers.
> 
> An existing jtreg test has been updated to reproduce this issue and verify the fix.

Jaikiran Pai has updated the pull request incrementally with one additional commit since the last revision:

  Update src/jdk.httpserver/share/classes/sun/net/httpserver/UnmodifiableHeaders.java
  
  Co-authored-by: Daniel Fuchs <67001856+dfuch at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19555/files
  - new: https://git.openjdk.org/jdk/pull/19555/files/c4713380..29a42d97

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19555&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19555&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19555/head:pull/19555

PR: https://git.openjdk.org/jdk/pull/19555