RFR: 8320577: Improve MessageHeader's toString() function to make HttpURLConnection's debug log readable [v2]

[Oliver Lockwood]

> At the moment, if you have debug logging turned on for the `HttpURLConnection` class, then from [line 746](https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/net/www/protocol/http/HttpURLConnection.java#L746) a log message like this is generated:
>> sun.net.www.MessageHeader at 49c4f2906 pairs: {null: HTTP/1.1 200}{Content-Type: application/json}{Transfer-Encoding: chunked}{Date: Thu, 09 Nov 2023 11:50:00 GMT}{Keep-Alive: timeout=60}{Connection: keep-alive}
> 
> The lack of separation after the `Object.toString()` output makes it hard to realise from the above that it's trying to say "6 pairs". 
> The change proposed by this PR would make the same log operation output this, instead:
>> sun.net.www.MessageHeader at 49c4f290 6 pairs: {null: HTTP/1.1 200}{Content-Type: application/json}{Transfer-Encoding: chunked}{Date: Thu, 09 Nov 2023 11:50:00 GMT}{Keep-Alive: timeout=60}{Connection: keep-alive}

Oliver Lockwood has updated the pull request incrementally with one additional commit since the last revision:

  Amend copyright header as requested by reviewers

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16615/files
  - new: https://git.openjdk.org/jdk/pull/16615/files/5cb93c0f..029cd9a4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16615&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16615&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16615.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16615/head:pull/16615

PR: https://git.openjdk.org/jdk/pull/16615