RFR: 8256363: Define toString() for MGF1ParameterSpec

[Weijun Wang]

Without this method, `PSSParameterSpec::toString` shows something like:
MD: SHA-256
MGF: java.security.spec.MGF1ParameterSpec at 77b52d12
SaltLength: 32
TrailerField: 1
This is ugly.

Noreg-trivial.

-------------

Commit messages:
 - 8256363: Define toString() for MGF1ParameterSpec

Changes: https://git.openjdk.java.net/jdk/pull/1208/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1208&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256363
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1208.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1208/head:pull/1208

PR: https://git.openjdk.java.net/jdk/pull/1208