ContentType seems to confuse OPC UA

The following code tries to read an OPC UA property

```
const read1 = await thing.readProperty("1:Speed");
console.log("speed value is: ", await read1.value());
```

with the following TD

```
"properties": {
"1:Speed": {
"title": "Speed",
"type": "number",
"forms": [
{
"href": "/?id=ns=1;s=MotorSpeed"
}
],
"observable": true
}
}
```

everything works fine while adding a `"contentType": "application/octet-stream"` which looks like this breaks it.

```
"properties": {
"1:Speed": {
"title": "Speed",
"type": "number",
"forms": [
{
"href": "/?id=ns=1;s=MotorSpeed",
"contentType": "application/octet-stream"
}
],
"observable": true
}
}
```

fails with

```
TypeError: dataType.toLowerCase is not a function
at OctetstreamCodec.valueToBytes
```

This seems to indicate that OPC UA data is processed by OctetstreamCodec also (again?)