#751 - Align Hop header assertions with HttpHeaders and javadocs.

Spring Framework accepts `null` for header values in HttpHeaders. This is already reflected in the javadocs.

Author: izeye

src/main/java/org/springframework/hateoas/client/Hop.java

@@ -95,8 +95,7 @@ public Hop withParameter(String name, Object value) {
 	 */
 	public Hop header(String headerName, String headerValue) {
 
-		Assert.notNull(headerName, "headerName must not be null!");
-		Assert.notNull(headerValue, "headerValue must not be null!");
+		Assert.hasText(headerName, "headerName must not be null or empty!");
 
 		if (this.headers == HttpHeaders.EMPTY) {