When user drags inactive object, make sure he won't scale it instead

To replicate the issue using current release of FabricJS:
1. Open http://fabricjs.com/.
2. Move your mouse pointer **exactly** to the corner of a red rectangle.
3. Do **not** select the rectangle, note that 'move' cursor is visible.
4. Drag the rectangle.

I would expect that object would be moved, but instead it's scaled, as exactly under the mouse pointer there is a control point (invisible at the beginning, until the object is selected).

The problem seems to be here:
https://github.com/kangax/fabric.js/blob/818ab118b30a9205a0e57620452b08bb8f5f18cc/src/mixins/canvas_events.mixin.js#L414-L415

`_findTargetCorner` checks whether the target is active (what makes sense). It's called twice, once in `_beforeTransform` and the second time in `_setupCurrentTransform`. However the target is activated in the meantime, in `_beforeTransform`. So the second call to `_findTargetCorner` actually returns corner and scaling is being activated. I've just moved target activation out of `_beforeTransform` and it seems to fix the problem for me. These functions are used only in this context, so hopefully it's not a risky change.

Regards,
Piotr