Preserve double 'overflow' declaration

[jmwebservices]

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).