RFR: 8277313: Validate header failed for test/jdk/java/net/httpclient/HeadTest.java

[Fairoz Matte]

With the integration of JDK-8276559, validate header failed as there is one missing comma after 2021 in `test/jdk/java/net/httpclient/HeadTest.java`

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Commit messages:
 - 8277313: Validate header failed for test/jdk/java/net/httpclient/HeadTest.java

Changes: https://git.openjdk.java.net/jdk/pull/6424/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6424&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277313
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6424.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6424/head:pull/6424

PR: https://git.openjdk.java.net/jdk/pull/6424