API: fix set milestone on PR creation (#14981)

* API: fix set milestone on PR creation

pr creation via API failed with 404, because we searched
for milestoneID 0, due to uninitialized var usage D:

* add tests

* fix expected status codes

* fix tests

Co-authored-by: 6543 <6543@obermui.de>

models/fixtures/repository.yml

@@ -148,6 +148,7 @@
   num_closed_issues: 0
   num_pulls: 1
   num_closed_pulls: 0
+  num_milestones: 1
   is_mirror: false
   num_forks: 1
   status: 0
@@ -734,4 +735,4 @@
   num_watches: 0
   num_projects: 0
   num_closed_projects: 0
-  status: 0
+  status: 0