logr.Marshaler: does not handle non-string results

[pohly]

/kind bug

What steps did you take and what happened:

I wrote a MarshalLog implementation which returns a slice. klog called the function, but then logged the original value instead of the one returned by MarshaLog.

What did you expect to happen:

klog/internal/serialize/keyvalues.go

Line 148 in 0990e81

writeStringValue(b, false, fmt.Sprintf("%+v", v))

should log value, not v.

[harshanarayana]

/assign