RFR: 8309118: HttpClient: Add more tests for 100 ExpectContinue with HTTP/2 [v2]

[Conor Cleary]

> ### **Issue Description**
> There is missing test coverage for the implementation of partial/informational responses (status code 1xx in server response headers) for the `HttpClient`, specifically in the `HTTP/2` use case. RFC 9113 - HTTP/2, details the behavior of any client in these situations. Small changes and new test cases are needed to verify compliance with this specification.  This issue primarily concerns how the client reacts to receiving a `RST_STREAM` frame at various stages of a partial/informational response from a server.
> 
> ### **Solution Details**
> Minor changes were made in `src/java.net.http/share/classes/jdk/internal/net/http/Stream.java` to improve the handling client's handling of receiving a `RST_STREAM`. `incoming_reset()` (L574) will now cause the client to ignore a `RST_STREAM` frame if an `END_STREAM` flag has been received _and_ the request body has completed sending. Previously it would be ignored only if an `END_STREAM` flag was seen which caused cancelled partial responses to 'hang' indefinitely should a client be transmitting data in a POST/PUT request. Some changes where also made to the `schedule()` method in Stream (L190) to ensure both the sending of the Request Body and receipt of a RST_STREAM at various stages of an exchange do not cause unexpected behavior.
> 
> Minor changes were made to the `Http2TestServer` implementation to improve the convinience of testing edge cases involving the sending of `HTTP/2` response headers. 
> 
> Concerning the new test cases, I have listed below the specifics of each case and the expected behavior of the client in the given scenario.
> 
> **test/jdk/java/net/httpclient/ExpectContinueTest.java**
> - Client sends a POST request with the `Expect: 100-Continue` header included
>    - Server responds with a `HEADERS` frame including a 100 status code and reads the request body. Server then sends `RST_STREAM` with code `NO_ERROR` or `PROTOCOL_ERROR` set.
>        - Expected/Observed Client Behavior: Client completes exceptionally in both cases.
> - Client sends a POST request with the `Expect: 100-Continue` header included
>    - Server responds with a `HEADERS` frame including a 100 status code and reads the request body. Server then sends Response Headers with status code 200 to complete the response. Server then sends RST_STREAM with code `NO_ERROR` or `PROTOCOL_ERROR` set.
>        - Expected/Observed Client Behavior: Client completes exceptionally in both cases.
> - Client sends a POST request with the `Expect: 10...

Conor Cleary has updated the pull request incrementally with one additional commit since the last revision:

  8309118: Cleanup identifiers and whitespace

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15664/files
  - new: https://git.openjdk.org/jdk/pull/15664/files/1993ecd6..a2fec5f3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15664&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15664&range=00-01

  Stats: 29 lines in 2 files changed: 4 ins; 9 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/15664.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15664/head:pull/15664

PR: https://git.openjdk.org/jdk/pull/15664