RFR: 8352502: Response message is null if expect 100 assertion fails with non 100 [v2]

[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.

Darragh Clarke has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - removed unneeded library
 - more cleanup
 - moved test, implemented feedback
 - merged master into branch
 - implementing comments
 - Response Message could be null

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25999/files
  - new: https://git.openjdk.org/jdk/pull/25999/files/10563157..d6f27780

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25999&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25999&range=00-01

  Stats: 113948 lines in 2239 files changed: 68346 ins; 28980 del; 16622 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