RFR: 8049520: FileCredentialsCache loads cache once and is never refreshed

[Weijun Wang]

Just re-load the CredentialsCache like we did on non-Windows non-macOS systems.

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Commit messages:
 - 8049520: FileCredentialsCache loads cache once and is never refreshed

Changes: https://git.openjdk.java.net/jdk/pull/5960/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5960&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8049520
  Stats: 73 lines in 2 files changed: 65 ins; 6 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5960.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5960/head:pull/5960

PR: https://git.openjdk.java.net/jdk/pull/5960