Be more consistent about types

For more details see https://github.com/videojs/videojs-contrib-hls/issues/103
If you're up for it, that would be great! I think it may also be necessary to modify that line of code you cited above:
```js
// if there is plenty of content in the buffer and we're not
// seeking, relax for awhile
if (typeof offset !== 'number' &&
  // ...
```