RFR: 8349670: HttpServer: sending interim responses fails after JDK-7026262 [v18]

[Josiah Noel]

> Following the guideline of the last comment on [JDK-8349670](https://bugs.openjdk.org/browse/JDK-8349670?focusedId=14794649&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14794649), resolves the issue where sending a 1xx status code would close the input stream, preventing the server from reading the body. 
> 
> - When a 1xx status code is sent by `sendResponseHeaders`, the input/output streams will not be closed prematurely.
> - sentHeaders will not be set to true when sending 1xx status codes
> - 100-continue will be sent automatically when trying to read the inputstream if `Expect: 100-continue` header is present

Josiah Noel has updated the pull request incrementally with two additional commits since the last revision:

 - Update Send1xxResponsesTest.java
 - pr comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27069/files
  - new: https://git.openjdk.org/jdk/pull/27069/files/e1ce09d9..f9dc2650

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27069&range=17
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27069&range=16-17

  Stats: 91 lines in 1 file changed: 56 ins; 18 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/27069.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27069/head:pull/27069

PR: https://git.openjdk.org/jdk/pull/27069