Convert byte[] via text/plain to String

Fix #898

When the payload is `byte[]` ensure that the
String is the String representation, not the
toString() conversion result.

spring-cloud-stream-integration-tests/src/test/java/org/springframework/cloud/stream/config/TextPlainConversionTest.java

@@ -63,6 +63,16 @@ public class TextPlainConversionTest {
 		assertThat(received.getPayload()).isEqualTo("Foo{name='Bar'}");
 	}
 
+	@Test
+	public void testByteArrayConversionOnOutput() throws Exception {
+		testProcessor.output().send(MessageBuilder.withPayload("Bar".getBytes()).build());
+		@SuppressWarnings("unchecked")
+		Message<?> received = ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class))
+				.messageCollector().forChannel(testProcessor.output()).poll(1, TimeUnit.SECONDS);
+		assertThat(received).isNotNull();
+		assertThat(received.getPayload()).isEqualTo("Bar");
+	}
+
 	@Test
 	public void testTextPlainConversionOnInputAndOutput() throws Exception {
 		testProcessor.input().send(MessageBuilder.withPayload(new Foo("Bar")).build());