Currently, methods are sorted by name. This happens to guarantee that
exported ASCII methods appear before non-exported ASCII methods, but
this breaks down when Unicode method names are considered.

Type.Method already accounts for this by always indexing into the
slice returned by exportedMethods. This CL makes Value.Method do the
same.

Fixes #22073.

Change-Id: I9bfc6bbfb7353e0bd3c439a15d1c3da60d16d209
Reviewed-on: https://go-review.googlesource.com/66770
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>