RFR: 8288109: HttpExchangeImpl.setAttribute does not allow null value after JDK-8266897

[Daniel Jeliński]

Please review this fix for a regression in HttpExchange's setAttribute method.

The specification of setAttribute explicitly states that null values are allowed.
JDK-8266897 changed `attributes` to `ConcurrentHashMap`, which does not allow null values.

This patch modifies `setAttribute` to remove the attribute from the map when null value is requested.

A new test was added to verify that setting attributes works as expected both for null and non-null values.

Tier1-3 clean.

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Commit messages:
 - Fix null attribute handling

Changes: https://git.openjdk.org/jdk/pull/13264/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13264&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288109
  Stats: 112 lines in 2 files changed: 111 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13264.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13264/head:pull/13264

PR: https://git.openjdk.org/jdk/pull/13264