Create a new spy object.

It can replace existing method if `obj` and `method` are
supplied.

@param {Object} obj (optional)
@param {String} method (optional)
@returns {Function} test double
@api public
function spy(obj, method) {
  if (!obj && !method) {
    obj = { spy: function(){} };
    method = 'spy';
  }
  return double(obj, method);
}