[binding-http] Sending the first GET request to observe a property starts logging of the TD to the console

## Version

core: 0.7.0
binding-http: 0.7.0
installed via npm

## Behavior

On the Thing side, every second I write to a property that updates its value with a function like this:

```js
private pollSensors(){
setInterval(()=>{
bme280.readSensorData().then((data) => {

this.thing.writeProperty("temperature",data.temperature_C)
this.thing.writeProperty("pressure",data.pressure_hPa)

});
},1000)
}
```
When the Thing starts, it logs the sensor values every second like the following

```
ExposedThing 'RainbowHAT' directly sets Property 'temperature' to value '36.81'
ExposedThing 'RainbowHAT' directly sets Property 'pressure' to value '949.9067119739695'
```
So far so good. Sending an HTTP GET request to read a property (`/properties/temperature`) results in the TD being logged to the console. This is weird but maybe it is OK.

However, sending a GET request to observe a property (`/properties/temperature/observable`) results in the TD being logged every second or every time the property is written to.