function createTransferStrategy(mergeStrategy) {
  return function (props, key, value) {
    if (!props.hasOwnProperty(key)) {
      props[key] = value;
    } else {
      props[key] = mergeStrategy(props[key], value);
    }
  };
}
Creates a transfer strategy that will merge prop values using the supplied
`mergeStrategy`. If a prop was previously unset, this just sets it.

@param {function} mergeStrategy
@return {function}