RFR: 8352502: Response message is null if expect 100 assertion fails with non 100 [v3]

[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 incrementally with one additional commit since the last revision:

  adding salting and path verification to test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25999/files
  - new: https://git.openjdk.org/jdk/pull/25999/files/d6f27780..0d826d75

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25999&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25999&range=01-02

  Stats: 79 lines in 1 file changed: 33 ins; 24 del; 22 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