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

[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}

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Commit messages:
 - Improve MessageHeader's toString() function to make HttpURLConnection's debug log readable

Changes: https://git.openjdk.org/jdk/pull/16615/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16615&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320577
  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