Activate specs for issue 623

This PR activates the specs for normalized decimal arguments in IE keyword args (sass/libsass#623) in preparation for sass/libsass#681.

This spec was previously activated in ab29d24e3cc65fb0298ac877d2ded0f3104b4549 and later reverted in 948b1c6fc7288f2c7b08ff59e642fad876c87b27 after I changed my mind on the implementation.

I've since reconsidered.