Can't remove only class when class merging disabled

Nobody has claimed this yet.

As [discussed on the mailing list](https://groups.google.com/d/topic/liftweb/qN4VThXAsXw/discussion):

Using Lift 2.5-M3, with the example from issue #1312, the following code removes the "removeme" class, as expected:

```
(".removeme !!" #> ("td [class!]" #> "removeme")).apply(Hi)
```

Produces:

```
NodeSeq(Hi)
```

However, if the "removeme" class is the only class, it is not removed:

```
(".removeme !!" #> ("td [class!]" #> "removeme")).apply(Hi)
```

Produces:

```
NodeSeq(Hi)
```

The consensus seems to be that this is a bug.

Thanks,
Kevin

Start by reproducing the two Lift selector examples in the issue, comparing removal when the td has multiple classes versus only "removeme". Locate the implementation behind the "td [class!]" transformation and add a regression test showing that the sole class is removed, then verify the output no longer contains the class attribute or value.

Written by the indexing model from the issue text.