Default plan branch is not exported to YAML

Issue Summary

When plan overrides default branch of linked repository YAML export doesn't generate correct code.

Steps to Reproduce

1. Create linked repository
2. Add repository to plan and override default branch
3. Export plan to YAML

Expected Results

---
version: 2
plan:
  project-key: PROJ
  key: PLAN
  name: My plan
  master-branch: production
stages:
...

Actual Results

---
version: 2
plan:
  project-key: PROJ
  key: PLAN
  name: My plan
stages:
...

Workaround

Enter valid branch name when copy content of produced YAML code to repository file