Fixed error when proxy_url is None.
Fixed error when proxy_url is None.

samuelchong/libcloud,vongazman/libcloud,lochiiconnectivity/libcloud,NexusIS/libcloud,DimensionDataCBUSydney/libcloud,SecurityCompass/libcloud,samuelchong/libcloud,ByteInternet/libcloud,samuelchong/libcloud,ZuluPro/libcloud,StackPointCloud/libcloud,DimensionDataCBUSydney/libcloud,ByteInternet/libcloud,ZuluPro/libcloud,mistio/libcloud,erjohnso/libcloud,vongazman/libcloud,apache/libcloud,mistio/libcloud,SecurityCompass/libcloud,Kami/libcloud,watermelo/libcloud,pquentin/libcloud,iPlantCollaborativeOpenSource/libcloud,mgogoulos/libcloud,techhat/libcloud,wuyuewen/libcloud,mgogoulos/libcloud,watermelo/libcloud,andrewsomething/libcloud,illfelder/libcloud,iPlantCollaborativeOpenSource/libcloud,pquentin/libcloud,techhat/libcloud,t-tran/libcloud,iPlantCollaborativeOpenSource/libcloud,lochiiconnectivity/libcloud,Scalr/libcloud,andrewsomething/libcloud,ZuluPro/libcloud,andrewsomething/libcloud,Kami/libcloud,illfelder/libcloud,vongazman/libcloud,NexusIS/libcloud,DimensionDataCBUSydney/libcloud,wuyuewen/libcloud,pquentin/libcloud,apache/libcloud,erjohnso/libcloud,SecurityCompass/libcloud,t-tran/libcloud,illfelder/libcloud,StackPointCloud/libcloud,mgogoulos/libcloud,techhat/libcloud,ByteInternet/libcloud,erjohnso/libcloud,lochiiconnectivity/libcloud,Scalr/libcloud,NexusIS/libcloud,watermelo/libcloud,wuyuewen/libcloud,mistio/libcloud,Scalr/libcloud,t-tran/libcloud,apache/libcloud,Kami/libcloud,StackPointCloud/libcloud