add opt-in non-coercing of strings for toArray

The current implementation of `toArray` coerces `String`s into `Array`s of characters. These commits adds an opt-in flag directing it to leave strings in tact.

Given:

```js
var array;
var string = 'a string';
```

allows code such as:
```js
if (! _.isArray(string)) {
  array = [string];
}
```

to be simplified to:
```js
array = _.toArray(string, true);
```

resulting in:
```js
['a string']
```

instead of (currently):
```js
['a', ' ', 's', 't', 'r', 'i', 'n', 'g']
```