encoding/gob: marshal maps using reflect.Value.MapRange

golang.org/cl/33572 added a map iterator.

use the reflect.Value.MapRange to fix map keys that contain a NaN

Fixes #24075

Change-Id: I0214d6f26c2041797703e48eac16404f189d6982
GitHub-Last-Rev: 5c01e117f4451dbaec657d02d006905df1d0055d
GitHub-Pull-Request: golang/go#47476
Reviewed-on: https://go-review.googlesource.com/c/go/+/338609


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>