Preserve double 'overflow' declaration

CSSO strips all but the last declaration of `overflow` within a declaration block. So,
```
.selector
{
overflow: auto;
overflow: overlay;
}
```
becomes the following:
```
.selector
{
overflow: overlay;
}
```
Declaring `overflow` twice is needed for UAs that do not support the `overlay` value.

This was tested with CSSO v3.5.0 and with 4.0.3 (online).

