RFR: 8285521: Minor improvements in java.net.URI

[Сергей Цыпанов]

- use `String.equalsIgnoreCase()` instead of hand-written code relying on `String.charAt()`
- use `String.compareToIgnoreCase()` instead of hand-written code relying on `String.charAt()`
- drop branches that are never executed
- drop unused argument from `URI.resolvePath()`
- simplify String-related operations

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Commit messages:
 - 8285521: Minor improvements in java.net.URI
 - 8285521: Minor improvements in java.net.URI

Changes: https://git.openjdk.java.net/jdk/pull/8397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8397&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285521
  Stats: 50 lines in 1 file changed: 0 ins; 34 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8397/head:pull/8397

PR: https://git.openjdk.java.net/jdk/pull/8397