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.
    *