Return chained image_tag to fix "size" attribute handling.

A call was originally being made to `image_tag_without_retina`, but the value was discarded and then it returned `tag("img")` which does not have the same effect. This changes the return to be the actual call to `image_tag_without_retina`. This has a side-effect of fixing https://github.com/davydotcom/retina_tag/issues/13 .

Tested, old output:
```
<img alt="..." class="..." src="..." size="21x21">
```

New output:
```
<img alt="..." class="..." src="..." width="21" height="21">
```

@davydotcom