Prevent clearing label background when it's set.

I.e. prevent clearing `label.backgroundColor` when it's neither unset
nor white.
Version 0.3.5
support for UITableView 'separatorColor' property
Revert "Merge branch 'UITableViewSeparator'"

This reverts commit 0e76060d64aeac0d01d59001a65ef5ddc3aba886, reversing
changes made to 3a0d2d34e292a0291e30e4e6fb4bdf82dc59db5e.