Previously fmt would first obtain a list of map keys
and then look up the value for each key. Since NaNs can
be map keys but cannot be fetched directly, the lookup would
fail and return a zero reflect.Value, which formats as <nil>.

golang.org/cl/33572 added a map iterator to the reflect package
that is used in this CL to retrieve the key and value from
the map and prints the correct value even for keys that are not
equal to themselves.

Fixes #14427

Change-Id: I9e1522959760b3de8b7ecf7a6e67cd603339632a
Reviewed-on: https://go-review.googlesource.com/129777Reviewed-by: Alan Donovan <adonovan@google.com>