Set extension regression (gogo#577)

* extensionsBytes set extension regression test

* fixed regression from commit 44af720 - using extensionsBytes and doing the following caused GetExtension to return the 'old' value. set -> get -> modify -> set -> get. The 'old' value was not cleared from the byteslice and getting returns the first extension of the given type.

proto/extensions.go

@@ -527,6 +527,7 @@ func ExtensionDescs(pb Message) ([]*ExtensionDesc, error) {
 // SetExtension sets the specified extension of pb to the specified value.
 func SetExtension(pb Message, extension *ExtensionDesc, value interface{}) error {
 	if epb, ok := pb.(extensionsBytes); ok {
+		ClearExtension(pb, extension)
 		newb, err := encodeExtension(extension, value)
 		if err != nil {
 			return err

test/setextensionbytes/Makefile

@@ -0,0 +1,31 @@
+# Protocol Buffers for Go with Gadgets
+#
+# Copyright (c) 2019, The GoGo Authors. All rights reserved.
+# http://github.com/gogo/protobuf
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+regenerate:
+	go install github.com/gogo/protobuf/protoc-gen-gogo
+	protoc --gogo_out=. --proto_path=../../../../../:../../protobuf/:. *.proto