RFR: 8263031: HttpClient throws Exception if it receives a Push Promise that is too large [v6]

[Conor Cleary]

> **Problem**
> When a Continuation Frame is received by the httpclient using HTTP/2 after a Push Promise frame (can happen if the amount of headers to be sent in a single Push Promise frame exceeds the maximum frame size, so a Continuation frame is required), the following exception occurs:
> 
> 
> java.io.IOException: no statuscode in response
> at java.net.http/jdk.internal.net.http.HttpClientImpl.send(HttpClientImpl.java:565)
> at java.net.http/jdk.internal.net.http.HttpClientFacade.send(HttpClientFacade.java:119)
> ...
> 
> This exception occurs because there is no existing flow in `jdk/internal/net/http/Http2Connection.java` which accounts for the case where a PushPromiseFrame is received with the END_HEADERS flag set to 0x0. When this occurs, the only acceptable frame/s (as multiple continuations are also acceptable) that can be received by the client on the same stream is a continuation frame.
> 
> **Fix**
> To ensure correct behavior, the following changes were made to `jdk/internal/net/http/Http2Connection.java`.
> 
> - The existing method `handlePushPromise()` was modified so that if the END_HEADERS flag is _unset_ (flags equal to 0x0), then a record used to track the state of the Push Promise containing a shared `HeaderDecoder` and the received `PushPromiseFrame` is initialised.
> - When the subsequent `ContinuationFrame` is received in `processFrame()`, the method `handlePushContinuation()` is called instead of the default flow resulting in `stream.incoming(frame)` being called (the source of the incorrect behaviour originally).
> - In `handlePushContinuation()`, the shared decoder is used to decode the received `ContinuationFrame` headers and if the `END_HEADERS` flag is set (flags equal to 0x4), the `HttpHeaders` object for the Push Promise as a whole is constructed which serves to combine the headers from both the `PushPromiseFrame` and the `ContinuationFrame`.
> 
> A regression test was included which verifies that the exception is not thrown and that the headers arrive correctly.

Conor Cleary has updated the pull request incrementally with two additional commits since the last revision:

 - 8263031: further verification of push promise and response
 - 8263031: Removed duplicate import

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7696/files
  - new: https://git.openjdk.java.net/jdk/pull/7696/files/c0543da9..55e2d993

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7696&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7696&range=04-05

  Stats: 48 lines in 2 files changed: 15 ins; 16 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7696.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7696/head:pull/7696

PR: https://git.openjdk.java.net/jdk/pull/7696