RFR: 8277606: String(String) constructor could copy hashIsZero

[PROgrm_JARvis]

This is a trivial fix to make `String(String)` constructor copy the value of `hashIsZero` field.

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Commit messages:
 - fix: copy `hashIsZero` in `String#String(String)`

Changes: https://git.openjdk.java.net/jdk/pull/6511/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6511&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277606
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6511.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6511/head:pull/6511

PR: https://git.openjdk.java.net/jdk/pull/6511