Fix broken filename when AWS Signature contains /

I'm using fog storage with AWS S3.

Sometimes [CarrierWave::Storage::Fog::File#url](http://www.rdoc.info/github/jnicklas/carrierwave/CarrierWave/Storage/Fog/File:url) returns url with Signature parameter that contains '/', something like:

`https://foobar.s3-ap-northeast-1.amazonaws.com/uploads/foobar/foobar/1/foobar.csv?AWSAccessKeyId=BLAHBLAHBLAH&Signature=EyLYyChcqFXEaCBg2p4M6/4LrWs=&Expires=1407927326`

the [CarrierWave::Storage::Fog::File#filename](http://www.rdoc.info/github/jnicklas/carrierwave/CarrierWave/Storage/Fog/File:filename) method returns `"4LrWs=&Expires=1407927326"` for this url :dizzy_face:

i fixed that.