encoding/textpb: fix handling of group field name

Group field name in textproto should be the type name.  Its field name
is derived from lowercasing its type name.

Change-Id: Ia12aafe934d3a59f3e07d09fe7939cfa6595a7b8
Reviewed-on: https://go-review.googlesource.com/c/157821
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/encoding/textpb/decode_test.go b/encoding/textpb/decode_test.go
index 00d8215..9911362 100644
--- a/encoding/textpb/decode_test.go
+++ b/encoding/textpb/decode_test.go
@@ -417,7 +417,7 @@
 		inputMessage: &pb2.Nests{},
 		inputText: `
 opt_nested: {}
-optgroup: {}
+OptGroup: {}
 `,
 		wantMessage: &pb2.Nests{
 			OptNested: &pb2.Nested{},
@@ -585,11 +585,11 @@
 		desc:         "repeated group fields",
 		inputMessage: &pb2.Nests{},
 		inputText: `
-rptgroup: {
+RptGroup: {
   rpt_bool: true
   rpt_bool: false
 }
-rptgroup: {}
+RptGroup: {}
 `,
 		wantMessage: &pb2.Nests{
 			Rptgroup: []*pb2.Nests_RptGroup{