code: public static Map<String, String> cleanupProperties(
              final Map<String, String> rawProperties) {
          return cleanupProperties(rawProperties, null);
      }
docs: Any properties which are not well known (i.e. in
  {@link BasicLTIConstants#validPropertyNames}) will be mapped to custom
  properties per the specified semantics. NOTE: no blacklisting of keys is
  performed.
  
  @param rawProperties A set of properties that will be cleaned.
  @return A cleansed version of rawProperties.