RFR: 8287390: Cleanup Map usage in AuthenticationInfo.requestAuthentication

[Andrey Turbanov]

`AuthenticationInfo.requestAuthentication` uses separate `HashMap`'s `get` +`put` calls.

            Thread t, c;
            c = Thread.currentThread();
            if ((t = requests.get(key)) == null) {
                requests.put (key, c);
                assert cached == null;
                return cached;
            }
            if (t == c) {
                assert cached == null;
                return cached;
            }


Instead we can use the `HashMap.putIfAbsent` to make code a bit easier to follow. We know that `requests` can contain only non-null values.

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Commit messages:
 - [PATCH] Cleanup Map usage in AuthenticationInfo.requestAuthentication

Changes: https://git.openjdk.java.net/jdk/pull/8484/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8484&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287390
  Stats: 10 lines in 1 file changed: 0 ins; 5 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8484.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8484/head:pull/8484

PR: https://git.openjdk.java.net/jdk/pull/8484