Merging two versions clears the original data in a single value custom field
Steps to Reproduce:
1. Create a custom field of type Single Version Picker named SVP
2. Create a project TEST
3. Create two versions in the project TEST named v1 and v2
4. Create 2 issues with SVP set to v1
5. Create 2 more issues with SVP set to v2
6. Go to the project TEST, and then Manage Versions
7. Choose Merge Versions and merge from v2 to v1
All 4 issues should have a value of v1 for SVP
The two issues which had a value of SVP of v2 now have not value set in the SVP field.
This also happens with the multi-version custom field type.