RFR: 8275079: Remove unnecessary conversion to String in java.net.http

[Andrey Turbanov]

Cleanup unnecessary String.valueOf calls (and similar) when conversion will happen implicitly anyway

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Commit messages:
 - 8275079: Remove unnecessary conversion to String in java.net.http
 - [PATCH] Remove unnecessary conversion to String

Changes: https://git.openjdk.java.net/jdk/pull/5795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5795&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275079
  Stats: 34 lines in 13 files changed: 0 ins; 3 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5795.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5795/head:pull/5795

PR: https://git.openjdk.java.net/jdk/pull/5795