Add getAuthenticateAsList to HttpHeaders

https://codereview.appspot.com/10450044/

Author: peleyal

google-http-client/src/main/java/com/google/api/client/http/HttpHeaders.java

@@ -786,6 +786,15 @@ public final String getAuthenticate() {
  return getFirstHeaderValue(authenticate);
  }
 
+ /**
+ * Returns all {@code "WWW-Authenticate"} headers or {@code null} for none.
+ *
+ * @since 1.16
+ */
+ public final List<String> getAuthenticateAsList() {
+ return authenticate;
+ }
+
  /**
  * Sets the {@code "WWW-Authenticate"} header or {@code null} for none.
  *