API delete branch: render branch name json instead of true

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Delete branch via API: doc updated

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'api-delete-branch-json' into 'master'

Api delete branch json

Fixes gitlab/gitlabhq#1478

See merge request !1233
Merge branch 'master' of github.com:gitlabhq/gitlabhq