RFR: 8288109: HttpExchangeImpl.setAttribute does not allow null value after JDK-8266897 [v2]

[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.

Daniel Jeliński has updated the pull request incrementally with one additional commit since the last revision:

  Use URIBuilder

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13264/files
  - new: https://git.openjdk.org/jdk/pull/13264/files/18f3a4f4..f35ab3ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13264&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13264&range=00-01

  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 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