RFR: 8352502: Response message is null if expect 100 assertion fails with non 100

[Darragh Clarke]

Currently if a request has set Expect-Continue and receives a non 100 response the `responseMessage` wouldn't be set.

This PR sets `responseMessage`, it also updates `getResponseMessage` to check if the message has already been set. This should match the way that `responseCode` is currently handled.

I also added a test to cover some possible responses.

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Commit messages:
 - Response Message could be null

Changes: https://git.openjdk.org/jdk/pull/25999/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25999&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352502
  Stats: 195 lines in 3 files changed: 195 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25999.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25999/head:pull/25999

PR: https://git.openjdk.org/jdk/pull/25999