Unset previously computed document change set if there are no changes

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no (afaik)
| Deprecations? | no
| Tests pass?   |  (waiting for travis..local testing resulted in unrelated errors)
| Fixed tickets | #297
| License       | MIT

See #297 for more details