EmbeddedJsonHeadersMessageMapperTests: race condition

The `EmbeddedJsonHeadersMessageMapperTests.testEmbedSome` does not embed a `timestamp` header.
To see that in action, add a short `Thread.sleep(2);` between
`encode` and `decode` operations.
Otherwise, there is no guarantee that the system clock might not produce the same timestamp
for a new message in the `EmbeddedJsonHeadersMessageMapper.toMessage()`

(cherry picked from commit 90b0555)

Author: artembilan

spring-integration-core/src/test/java/org/springframework/integration/support/json/EmbeddedJsonHeadersMessageMapperTests.java

@@ -51,6 +51,8 @@ public void testEmbedSome() throws Exception {
 		EmbeddedJsonHeadersMessageMapper mapper = new EmbeddedJsonHeadersMessageMapper(MessageHeaders.ID);
 		GenericMessage<String> message = new GenericMessage<>("foo");
 		byte[] encodedMessage = mapper.fromMessage(message);
+		// Give some delay for a new timestamp to be generated
+		Thread.sleep(2);
 		Message<?> decoded = mapper.toMessage(encodedMessage);
 		assertThat(decoded.getPayload()).isEqualTo(message.getPayload());
 		assertThat(decoded.getHeaders().getTimestamp()).isNotEqualTo(message.getHeaders().getTimestamp());