GH-3246: Fix NPEs for AMQP properties conversion

Fixes: #3246

The received message might come not from Spring AMQP source,
therefore it might not have `priority` and `content-type` set.

* Fix `DefaultMessagePropertiesConverter` checking for the nulls in the
`priority` and `content-type`
* Remove redundant null check for `priority` in the `MessageBuilderSupport`

Author: artembilan

spring-amqp/src/main/java/org/springframework/amqp/core/MessageBuilderSupport.java

@@ -31,6 +31,7 @@
  * @param <T> The message builder type.
  *
  * @author Gary Russell
+ * @author Artem Bilan
  *
  * @since 1.3
  *
@@ -267,8 +268,7 @@ public MessageBuilderSupport<T> setExpirationIfAbsent(String expiration) {
 	}
 
 	public MessageBuilderSupport<T> setPriorityIfAbsentOrDefault(Integer priority) {
-		if (this.properties.getPriority() == null
-				|| MessageProperties.DEFAULT_PRIORITY.equals(this.properties.getPriority())) {
+		if (MessageProperties.DEFAULT_PRIORITY.equals(this.properties.getPriority())) {
 			this.properties.setPriority(priority);
 		}
 		return this;

spring-rabbit/src/main/java/org/springframework/amqp/rabbit/support/DefaultMessagePropertiesConverter.java

@@ -127,8 +127,14 @@ else if (MessageProperties.RETRY_COUNT.equals(key)) {
 		}
 		target.setDeliveryMode(null);
 		target.setExpiration(source.getExpiration());
-		target.setPriority(source.getPriority());
-		target.setContentType(source.getContentType());
+		Integer priority = source.getPriority();
+		if (priority != null) {
+			target.setPriority(priority);
+		}
+		String contentType = source.getContentType();
+		if (contentType != null) {
+			target.setContentType(contentType);
+		}
 		target.setContentEncoding(source.getContentEncoding());
 		String correlationId = source.getCorrelationId();
 		if (StringUtils.hasText(correlationId)) {