Fix in clearing space fields of config data on cf space-delete

Before Fix: If cf user deleting the targeted space but with space name as
case insenstive then space fields are not deleted from the config
htipl@cf5:~/go/src/github.com/cloudfoundry/cli$ cf spaces
Getting spaces in org SriniOrg as admin...
name
No spaces found
htipl@cf5:~/go/src/github.com/cloudfoundry/cli$ cf target
API endpoint:   https://api.10.244.0.34.xip.io (API version: 2.13.0)
User:           admin
Org:            SriniOrg
Space:          SriniSpace

After Fix:If cf user does same steps
space fields are deleted from the config
htipl@cf5:~/go/src/github.com/cloudfoundry/cli$ ./out/cf spaces
Getting spaces in org SriniOrg as admin...
name
No spaces found
htipl@cf5:~/go/src/github.com/cloudfoundry/cli$ ./out/cf target
API endpoint:   https://api.10.244.0.34.xip.io (API version: 2.13.0)
User:           admin
Org:            SriniOrg
Space:          No space targeted, use 'cf target -s SPACE'

http://rnd-github.huawei.com/paas/cli/issues/28
Merge pull request #309 from HuaweiTech/hw-issue-28

Fix in clearing space fields of config data on cf space-delete