Indexing a String by property name should return an error in ErrorWhenMissing mode

A `LiquidString` should generate a rendering error when `ErrorWhenMissing()` is on, _and_ the property is not a `LiquidNumeric` value _and_ it isn't a "special" property like `.first`, `.last` or `.size`.

Should work:

``` liquid
{{ mystring.last }}
{{ mystring[0] }}
```

Should not work:

``` liquid
{{ mystring.foo }}
```
