modifiers: dimension are calculated incorrectly if they vary between elements

**Describe the bug**
If the sortable elements have different margins, ember-sortable calculates the height of the elements incorrectly, which collapses all margins.

**To Reproduce**
Use the following css to apply margins to sortable elements
`li + li { margin-top: 20px; }`

**Expected behavior**
Existing margins should not collapse.

**Additional context**
This issue has been noticed using the modifiers version of the add-on.
I tried to find a quick fix, however there seem to more issues within calculating the height of the element. For instance only marginBottom is applied, but no marginTop. Adding marginTop fixed some issue, but introduced new ones, so there probably is another bug with calculating the position correctly.