htv:headers not conveyed when exposing thing

When exposing a thing with http bindings, the `htv:headers` are emptied / not transported to the thing description of the exposed thing.

As an example, this is an excerpt of the td before exposing it:

```json
"properties": {
"counter": {
"type": "integer",
"writeOnly": false,
"readOnly": false,
"forms": [
{
"htv:headers": [
{
"htv:fieldName": "Cache-Control",
"htv:fieldValue": "max-age=0"
}
]
}
]
},
```

After exposing, a href is there, but `htv:headers` has disappeared.

I believe it should be possible to provide values here.