Can't set specific width and height on capture

Hi,

I can't manage to set width and height for capture. Here's my code:
```
cap = new cv.VideoCapture(options.videoDeviceId);

console.log(cap.get(cv.CAP_PROP_FRAME_WIDTH));
console.log(cap.get(cv.CAP_PROP_FRAME_HEIGHT));

# Apply new camera size
cap.set(cv.CAP_PROP_FRAME_WIDTH, 1280);
cap.set(cv.CAP_PROP_FRAME_HEIGHT, 720);

console.log(cap.get(cv.CAP_PROP_FRAME_WIDTH));
console.log(cap.get(cv.CAP_PROP_FRAME_HEIGHT));
```
Which gives me the following output:

```
640
480
640
480
```

Am I doing something wrong ?

PS: Normally, my webcam should support the resolution 1280x720:
```
lsusb -s 001:003 -v | egrep "Width|Height"
Couldn't open device, some information will be missing
wWidth 640
wHeight 360
wWidth 640
wHeight 480
wWidth 176
wHeight 144
wWidth 320
wHeight 240
wWidth 352
wHeight 288
wWidth 1280
wHeight 720
wWidth 640
wHeight 360
wWidth 640
wHeight 480
wWidth 176
wHeight 144
wWidth 320
wHeight 240
wWidth 352
wHeight 288
```

EDIT: I initially put a Coffeescript snippet, I changed it to Javascript.