cloud/storage: change User-Agent separator from ';' to ' '
cloud/storage: change User-Agent separator from ';' to ' '

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.43

Change-Id: Ia90d68f594a8f6b54cd0dd3d4947efa006974c57

ptomasroos/gcloud-golang,michael-grosshans/google-cloud-go,Maksadbek/gcloud-golang,googleapis/google-cloud-go,googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,googleapis/google-cloud-go,googleapis/google-cloud-go,blowmage/gcloud-golang,michael-grosshans/google-cloud-go,blowmage/gcloud-golang,hsinhoyeh/gcloud-golang,roth1002/gcloud-golang,Maksadbek/gcloud-golang,roth1002/gcloud-golang,ptomasroos/gcloud-golang