Date resolved is updated whenever resolution is included in transition screen

If the resolution field is included on a transition (for example, from resolved->closed), the resolution date is updated whether or not the value changes.

Steps to reproduce:

1. Create a new screen that includes the resolution field
2. Create a new workflow (based on the default, for example)
3. Edit the resolved->closed transition and associate it with the new screen
4. Create a new workflow scheme and associate the new workflow with all issue types
5. Configure a project to use the new workflow scheme
6. Resolve the issue, including a comment. Note the time of the comment and the resolved date.
7. Wait at least one minute.
8. Close the issue without changing the resolution. Include a comment. Note that the resolved date matches the time of the comment on the resolved->closed transition.

You could argue that changing the resolution should update the resolved date (although that's not entirely obvious), but simply including the resolution on a transition shouldn't force a change.