Route parameters and ES6 default parameters

This is an rather small issue with route parameters and ES6 default parameters that I just ran into and thought I would just bring it up (semi-related to #3828 I guess):

Say we have an Router that looks like this:

```
const Router = Backbone.Router.extend({
routes: {
"(:foo)(/)(:bar)": "biz"
}
});

let router = new Router();

router.on("route:init", (foo, bar) => {
console(foo, bar);
});
```

In [backbone.js#L1532](https://github.com/jashkenas/backbone/blob/c20ca2e3f6dc28aba0ff019d269650406a140c73/backbone.js#L1532) the comment above the `_extractParameters` method states:

> // Given a route, and a URL fragment that it matches, return the array of
> // extracted decoded parameters. Empty or unmatched parameters will be
> // treated as `null` to normalize cross-browser behavior.

When navigating to `foo.com`, we get the following output:

```
console.log(foo, bar) // null, null
```

There's nothing wrong with this, but it gets a bit more trickier when [default parameters](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/default_parameters) come in:

Say we want to use some default parameters if none are supplied, in ES5 we would do something like this:

```
router.on("route:init", (foo, bar) => {
foo ? foo : "hamburger";
bar ? bar : "vegan";
console(foo, bar);
});
```

In ES6, one could just use default parameters:

```
router.on("route:init", (foo = "hamburger", bar = "vegan") => {
console(foo, bar); // expected: hamburger, vegan
});
```

However, as MDN states:

> Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.

The output from that is still `null, null` because the passed in values are `null`, rendering default parameters useless.

```
router.on("route:init", (foo = "hamburger", bar = "vegan") => {
console(foo, bar); // null, null
});
```

Solution would be passing no value at all or undefined instead of null, but I am not sure which browsers the comment about cross-browser inconsistencies refers to.
The workarounds are rather trivial (and it might not be the best use-case in general either), so the issue is not that urgent by any means.

Start in backbone.js at the _extractParameters method and review the comment about empty or unmatched parameters being normalized to null, along with the linked route-parameter context. Determine the intended cross-browser behavior for missing values and ES6 defaults; the issue is done when that behavior is decided and the route callback receives the agreed value consistently.

Written by the indexing model from the issue text.