bug in measure the width for px unit

```ts 

    const doc = new jsPDF({
      unit: 'px',
      format: 'a4', // ensures proper A4 size
    }); 
    const pageWidth2 = doc.internal.pageSize.width;
  console.log(doc.internal.pageSize.height , doc.internal.pageSize.width);

```
the res is     631.4175     446.46
    

also i test other unit they are work as well 
