spanner/spannertest: handle returning DATE values through RPC interface

The mapping to the correct Spanner type code was overlooked.

Change-Id: I9118caf02e43fcdaa98ca42f90deb0c931fcc41e
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/44110
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Knut Olav Løite <koloite@gmail.com>

spanner/spannertest/inmem.go

@@ -678,6 +678,8 @@ func spannerTypeFromType(typ spansql.Type) (*spannerpb.Type, error) {
 		code = spannerpb.TypeCode_STRING
 	case spansql.Bytes:
 		code = spannerpb.TypeCode_BYTES
+	case spansql.Date:
+		code = spannerpb.TypeCode_DATE
 	}
 	st := &spannerpb.Type{Code: code}
 	if typ.Array {