Default values for string extensions break inheritance

Hi,
I use a structure like this:

``` apiblueprint
# Data Structures

## Date (string)
### Default: `2016-12-24T22:00:00.000Z`
```

See MSON specification: https://apiblueprint.org/documentation/mson/specification.html#45-default

and this seems to work, but only partially. It works for some endpoints, but then breaks and always displays "Hello, world!".

You can reproduce this by loading and rendering the following git repo (with that exact commit):
https://build.se.informatik.uni-kiel.de/iobserve-ui/iobserve-ui-api-docs/tree/1a99df93ae2cfb98b7541bc37190fae7fd015a71

In that repo it works until the `Communications` enpoints are reached, see property `lastUpdate`

Versions:

```
aglio 2.2.0
olio 1.6.2
```
## EDIT:

I noticed that I wasn't using my `Date` type consistently, instead it actually overrides the default value for **all** strings!
Therefore also overriding all other default values for types that extend string