RFR: 8280377: MethodHandleProxies does not correctly invoke default methods with varags

[Mandy Chung]

The MethodHandle of a default method should be made as a fixed arity method handle because it is invoked via Proxy's invocation handle with a non-vararg array of arguments.  On the other hand, the `InvocationHandle::invokeDefault` method  was added in Java 16 to invoke a default method of a proxy instance.  This patch simply converts the implementation to call `InvocationHandle::invokeDefault` instead.

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Commit messages:
 - 8280377: MethodHandleProxies does not correctly invoke default methods with varags

Changes: https://git.openjdk.java.net/jdk/pull/7185/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7185&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280377
  Stats: 49 lines in 2 files changed: 8 ins; 37 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7185.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7185/head:pull/7185

PR: https://git.openjdk.java.net/jdk/pull/7185