[jdk11u-dev] RFR: 8225583: Examine the HttpResponse.BodySubscribers for null handling and multiple subscriptions

[Zhengyu Gu]

A clean backport  for parity with Oracle 11.0.14. 

Test:
- [x]Passed test (test/jdk/java/net/httpclient/BodySubscribersTest.java) in followup backport (JDK-8226319)

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Commit messages:
 - Backport af2a46b094b389b8018870b8273a5283b5528395

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/174/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=174&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225583
  Stats: 35 lines in 3 files changed: 32 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/174.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/174/head:pull/174

PR: https://git.openjdk.java.net/jdk11u-dev/pull/174