Set string value for originalContentType in message header

- Since the `originalContentType` header is preserved across the wire while the serialization/de-serialization processes take place using the deduced `contentType`,
it is convenient to set the `String` value of the `originalContentType` instead of setting the actual `MimeType` object in the header.
 - Once the `de-serialization` has taken place, the deduced `contentType` is always replaced with the `originalContentType` as the `contentType` in the message header and this preserved contentType can be resolved to its corresponding MimeType using `ContentTypeResolver` anytime. Hence, using the `String` object for `originalContentType` doesn't do any harm.

 - Fix the issue while setting the deduced contentType based on the `payload` when the `originalContentType` is `application/json`
   - Currently, the contentType is set to `text/plain` which seems incorrect
 - Also, add the `application/json` contentType to return `String` object during de-serialization process.

This resolves #383

Use `text/plain` contentType when the payload is of type `String`

  - This is for the intermediate contentType that is used for serialization/de-serialization

spring-cloud-stream-binders/spring-cloud-stream-binder-test/src/test/java/org/springframework/cloud/stream/binder/MessageChannelBinderSupportTests.java

@@ -50,6 +50,7 @@ import com.esotericsoftware.kryo.Registration;
 /**
  * @author Gary Russell
  * @author David Turanski
+ * @author Ilayaperumal Gopinathan
  */
 public class MessageChannelBinderSupportTests {
 
@@ -122,11 +123,11 @@ public class MessageChannelBinderSupportTests {
 
 		assertEquals(MimeTypeUtils.TEXT_PLAIN,
 				contentTypeResolver.resolve(converted.getHeaders()));
-		assertEquals(MimeTypeUtils.APPLICATION_JSON,
+		assertEquals(MimeTypeUtils.APPLICATION_JSON.toString(),
 				converted.getHeaders().get(BinderHeaders.BINDER_ORIGINAL_CONTENT_TYPE));
 		MessageValues reconstructed = binder.deserializePayloadIfNecessary(converted);
 		assertEquals("{\"foo\":\"foo\"}", reconstructed.getPayload());
-		assertEquals(MimeTypeUtils.APPLICATION_JSON, reconstructed.get(MessageHeaders.CONTENT_TYPE));
+		assertEquals(MimeTypeUtils.APPLICATION_JSON.toString(), reconstructed.get(MessageHeaders.CONTENT_TYPE));
 	}
 
 	@Test