code: public String put(String key, String value, boolean percentEncode) {
          SortedSet<String> values = wrappedMap.get(key);
          if (values == null) {
              values = new TreeSet<String>();
              wrappedMap.put(percentEncode ? OAuth.percentEncode(key) : key, values);
          }
          if (value != null) {
              value = percentEncode ? OAuth.percentEncode(value) : value;
              values.add(value);
          }
  
          return value;
      }
docs: Convenience method to add a single value for the parameter specified by
  'key'.
  
  @param key
  the parameter name
  @param value
  the parameter value
  @param percentEncode
  whether key and value should be percent encoded before being
  inserted into the map
  @return the value