[heycam/webidl] Are string types and sequence types distinguishable? (#266)

As discussed in whatwg/url#204, there is some confusion about the distinguishability between string types and sequences. Although currently, the spec maintains that they are [distinguishable](https://heycam.github.io/webidl/#dfn-distinguishable), @domenic and @annevk have express doubts over that.

What I have found is that:

1. ES string primitives are **NOT** valid sequences, and are therefore distinguishable. Even though string primitives have a `@@iterator`, they fail the [`typeof === 'object'` check](https://heycam.github.io/webidl/#es-to-sequence) in conversion to a sequence.
2. ES string wrapper objects **ARE** valid sequences. They pass the aforementioned test, and they likewise have a valid iterator interface.

This is exemplified in Firefox, which implements iterable-to-sequence conversion:

```js
>> new Headers([ 'ab' ]).get('a')
x  "TypeError: Element of member of HeadersOrByteStringSequenceSequenceOrByteStringMozMap can't be converted to a sequence."
>> new Headers([ new String('ab') ]).get('a')
<- "b"
```

How can we resolve this situation?

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