should not insert version if all items were filtered

This is a behavioral issue that comes from doing a partial mirror. If all items in a version were filtered out, we are currently still inserting the version. That causes problems if you change your filter items later.

See example here:
$ dlfile=examples/foocloud/streams/v1/com.example.foovendor:released:download.json
$ rm -Rf out.d; ./tools/tenv ./bin/sstream-mirror -v $dlfile out.d/ arch=amd64
com.example.foovendor:released:download/com.example.foovendor:pinky:server:amd64: to_add=['20121026.1', '20121001', '20120328'] to_remove=[]
com.example.foovendor:released:download/com.example.foovendor:pinky:server:i386: to_add=['20121026.1', '20121001', '20120328'] to_remove=[]

$ ./tools/tenv ./bin/sstream-mirror -v $dlfile out.d/ arch=amd64
com.example.foovendor:released:download/com.example.foovendor:pinky:server:i386: to_add=[] to_remove=[]
com.example.foovendor:released:download/com.example.foovendor:pinky:server:amd64: to_add=[] to_remove=[]

$ ./tools/tenv ./bin/sstream-mirror -v $dlfile out.d/
com.example.foovendor:released:download/com.example.foovendor:pinky:server:amd64: to_add=[] to_remove=[]
com.example.foovendor:released:download/com.example.foovendor:pinky:server:i386: to_add=[] to_remove=[]

$ find out.d/ -type f
out.d/files/beta-2/foovendor-6.1-beta2-server-cloudimg-amd64-disk1.img
out.d/files/beta-2/foovendor-6.1-beta2-server-cloudimg-amd64-root.tar.gz
out.d/files/beta-2/foovendor-6.1-beta2-server-cloudimg-amd64.tar.gz
out.d/files/release-20121026.1/foovendor-6.1-server-cloudimg-amd64.tar.gz
out.d/files/release-20121026.1/foovendor-6.1-server-cloudimg-amd64-root.tar.gz
out.d/files/release-20121026.1/foovendor-6.1-server-cloudimg-amd64-disk1.img
out.d/files/release-20121001/foovendor-6.1-server-cloudimg-amd64.tar.gz
out.d/files/release-20121001/foovendor-6.1-server-cloudimg-amd64-root.tar.gz
out.d/files/release-20121001/foovendor-6.1-server-cloudimg-amd64-disk1.img
out.d/.data/com.example.foovendor:released:download
out.d/streams/v1/com.example.foovendor:released:download.json

Note, it thinks it already has i386 stuff, and the .data/...download file agrees, with a section like:
  "com.example.foovendor:pinky:server:i386": {
   "arch": "i386",
   "stream": "released",
   "versions": {
    "20120328": {
     "label": "beta2"
    },
    "20121026.1": {
     "label": "release"
    },
    "20121001": {
     "label": "release"
    }
   },
   "release": "pinky",
   "build": "server",
   "version": "6.1"
  }

ProblemType: Bug
DistroRelease: Ubuntu 13.10
Package: simplestreams 0.1.0~bzr315-0ubuntu1
ProcVersionSignature: Ubuntu 3.11.0-9.16-generic 3.11.2
Uname: Linux 3.11.0-9-generic x86_64
ApportVersion: 2.12.5-0ubuntu1
Architecture: amd64
Date: Thu Oct 10 15:36:16 2013
EcryptfsInUse: Yes
InstallationDate: Installed on 2011-10-19 (722 days ago)
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 TERM=screen-bce
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: simplestreams
UpgradeStatus: Upgraded to saucy on 2013-05-20 (143 days ago)