HttpHeaders fails getAllow if set to EmptyCollection

Prior to this commit, calls to getAllow would fail is setAllow was set
to an EmptyCollection right before.

    java.lang.IllegalArgumentException: No enum constant
    org.springframework.http.HttpMethod

This commit fixes this by testing the header value for an empty value
before trying to use it to get a value from the Enum.

Issue: SPR-11917

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -224,7 +224,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	 */
 	public Set<HttpMethod> getAllow() {
 		String value = getFirst(ALLOW);
-		if (value != null) {
+		if (!StringUtils.isEmpty(value)) {
 			List<HttpMethod> allowedMethod = new ArrayList<HttpMethod>(5);
 			String[] tokens = value.split(",\\s*");
 			for (String token : tokens) {