function once(func) {
      return before(2, func);
    }
Creates a function that is restricted to invoking `func` once. Repeat calls
to the function return the value of the first invocation. The `func` is
invoked with the `this` binding and arguments of the created function.

@static
@memberOf _
@since 0.1.0
@category Function
@param {Function} func The function to restrict.
@returns {Function} Returns the new restricted function.
@example

var initialize = _.once(createApplication);
initialize();
initialize();
// => `createApplication` is invoked once