The issue here isn't whether or not Syria has WMDs. If we were to attack every country who has WMDs then we'd be off to war against Russia, China, and many of our most closest allies. Hell, we would have to bomb ourselves since we are one of the world's leaders in WMDs!
The issue is Asswad using them. Now what proof do we have that he even used them? Has the UN report proven that yet or does it have yet to come in? Now why would Asswad even need to use them? How do we know that Al-Queda didn't obtain them and use them in some Syrians? Or framed Asswad? How do we know for sure that they were even used?
To me, lets call their bluff. If we do nothing, then their plan to have us do their fighting against Asswad for them will have backfired. If we do nothing but it really was Asswad? Meh. Who cares? As long as he doesn't do it again who cares? Ultimately, our message was sent and received. If we do nothing and he does it again, then we will have a surety that he really is a jerk face and maybe just maybe someone closer to the situation will fix him for us. Does Putin really want to be seen by the international community as a supporter of a serial gasser?