Fix creation of push rules via POST API

If the `max_file_size` parameter were not specified, Grape would
default that value to `nil`, which prevents the model from being
validated and saved. As a result, users would see a JSON response
that had an `id` of `nil`, and the push rule was never created.

Closes #2820

changelogs/unreleased-ee/sh-fix-post-push-rule.yml

+---
+title: Fix creation of push rules via POST API
+merge_request:
+author:

lib/api/project_push_rule.rb

@@ -42,7 +42,7 @@ module API
         if user_project.push_rule
           error!("Project push rule exists", 422)
         else
-          push_rule = user_project.create_push_rule(declared_params)
+          push_rule = user_project.create_push_rule(declared_params(include_missing: false))
           present push_rule, with: Entities::ProjectPushRule
         end
       end

spec/requests/api/project_push_rule_spec.rb

@@ -59,6 +59,16 @@ describe API::ProjectPushRule, 'ProjectPushRule', api: true  do
       end
     end
 
+    it 'adds push rule to project with no file size' do
+      post api("/projects/#{project.id}/push_rule", user),
+        commit_message_regex: 'JIRA\-\d+'
+
+      expect(response).to have_http_status(201)
+      expect(json_response['project_id']).to eq(project.id)
+      expect(json_response['commit_message_regex']).to eq('JIRA\-\d+')
+      expect(json_response['max_file_size']).to eq(0)
+    end
+
     it 'returns 400 if no parameter is given' do
       post api("/projects/#{project.id}/push_rule", user)