encoding/textpb: ignore reserved field names during unmarshal

Do not treat reserved field names as unknown. Skip over these instead
of returning error for unknown field.

Change-Id: Iba9371fd901a0690e5b7329bccf261570352b55d
Reviewed-on: https://go-review.googlesource.com/c/154178
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/textpb/decode_test.go b/encoding/textpb/decode_test.go
index e501fe5..8b6025a 100644
--- a/encoding/textpb/decode_test.go
+++ b/encoding/textpb/decode_test.go
@@ -963,6 +963,11 @@
 			},
 		},
 		wantErr: true,
+	}, {
+		desc:         "ignore reserved field",
+		inputMessage: &pb2.Nests{},
+		inputText:    "reserved_field: 'ignore this'",
+		wantMessage:  &pb2.Nests{},
 	}}
 
 	for _, tt := range tests {