Fix HttpHeaderMapper to support MimeType

It turns out that `contentType` header (as well as `Accept`) may be
converted into `MimeType` upstream before the message is sent into HTTP
Outbound Channel Adapter meanwhile the logic is only to support String
and MediaType

* Change the `DefaultHttpHeaderMapper` to support more generic `MimeType`
for the `Content-Type` and `Accept` HTTP headers which can simply be
converted to the expected `MediaType`

**Cherry-pick to 5.1.x & 5.0.x**

# Conflicts:
#	spring-integration-http/src/main/java/org/springframework/integration/http/support/DefaultHttpHeaderMapper.java

Author: artembilan

spring-integration-http/src/main/java/org/springframework/integration/http/support/DefaultHttpHeaderMapper.java

@@ -55,6 +55,7 @@
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.MimeType;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.PatternMatchUtils;
 import org.springframework.util.StringUtils;
@@ -591,8 +592,8 @@ private void setHttpHeader(HttpHeaders target, String name, Object value) {
 				if (!CollectionUtils.isEmpty(values)) {
 					List<MediaType> acceptableMediaTypes = new ArrayList<>();
 					for (Object type : values) {
-						if (type instanceof MediaType) {
-							acceptableMediaTypes.add((MediaType) type);
+						if (type instanceof MimeType) {
+							acceptableMediaTypes.add(MediaType.asMediaType((MimeType) type));
 						}
 						else if (type instanceof String) {
 							acceptableMediaTypes.addAll(MediaType.parseMediaTypes((String) type));
@@ -749,8 +750,8 @@ else if (value instanceof String) {
 			}
 		}
 		else if (MessageHeaders.CONTENT_TYPE.equalsIgnoreCase(name)) {
-			if (value instanceof MediaType) {
-				target.setContentType((MediaType) value);
+			if (value instanceof MimeType) {
+				target.setContentType(MediaType.asMediaType((MimeType) value));
 			}
 			else if (value instanceof String) {
 				target.setContentType(MediaType.parseMediaType((String) value));

spring-integration-http/src/test/java/org/springframework/integration/http/support/DefaultHttpHeaderMapperFromMessageOutboundTests.java

@@ -43,6 +43,7 @@
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
+import org.springframework.util.MimeType;
 import org.springframework.util.StopWatch;
 
 /**
@@ -345,6 +346,18 @@ public void validateContentTypeAsMediaType() {
 		assertEquals("html", headers.getContentType().getSubtype());
 	}
 
+	@Test
+	public void validateContentTypeAsMimeType() {
+		HeaderMapper<HttpHeaders> mapper = DefaultHttpHeaderMapper.outboundMapper();
+		Map<String, Object> messageHeaders = new HashMap<>();
+		messageHeaders.put(MessageHeaders.CONTENT_TYPE, new MimeType("text", "plain"));
+		HttpHeaders headers = new HttpHeaders();
+
+		mapper.fromHeaders(new MessageHeaders(messageHeaders), headers);
+		assertEquals("text", headers.getContentType().getType());
+		assertEquals("plain", headers.getContentType().getSubtype());
+	}
+
 	// Date test
 
 	@Test