ConstrainWithin(weight， hight) , Is it the hight not force？

I code like this.
```go
func ImageThumb(sBuffer []byte, w float64, h float64) ([]byte, error) {
step := imageflow.NewStep()
data, err := step.Decode(
imageflow.NewBuffer(sBuffer),
// FIXME: hight is not force
).ConstrainWithin(w, h).Encode(imageflow.GetBuffer("thumb"), imageflow.MozJPEG{Quality: 60}).Execute()
if err != nil {
return nil, err
}
//ioutil.WriteFile(fmt.Sprintf("test_%fx%f.jpeg", w, h), data["thumb"], 0644)
return data["thumb"], nil
}
```
But I found , the 'hight' arg is not work well. When call this func, `ImageThumb(sButter, 80 , 80)`
Maybe I can get a image with size 80 * 40 , or any other hight , seems to depend on what the original picture‘s hight

Start by reproducing the ImageThumb example with ConstrainWithin(80, 80) and inspect the resulting image dimensions. Trace the Go binding's ConstrainWithin entry point to determine whether the aspect-ratio-preserving result is expected or indicates a defect. Done means confirming the intended height behavior and documenting or testing the result.

Written by the indexing model from the issue text.