Scalars: reject array serialization/coercion (#1336)
Working on #1333 I noticed that you can pass arrays as scalars in a query variable, e.g. [SWAPI example](http://graphql.org/swapi-graphql/?query=query%20(%24id%3A%20ID)%20%7B%0A%20%20person(personID%3A%20%24id)%20%7B%0A%20%20%20%20name%0A%20%20%7D%0A%7D&variables=%7B%0A%20%20%22id%22%3A%20%5B1%5D%0A%7D)
This PR is based on #925 but fixing same issue for types other than `String` e.g.
```js
Number([1]) // 1
```
