Meets #8419: [PDF Export] Internal error when entering wrong YAML

[``* `#8419`  [PDF Export] Internal error when entering wrong YAML``](https://www.openproject.org/work_packages/8419)
Each ``it`` in the ``describe 'Create'`` block shares the same setup with different parameters. I'd like to ask you to refactor that:
1. Create a shared example
2. Put ``post 'create', @params`` into a ``before`` block of that shared example
3. Put ``response.should render_template('new')`` into an ``it`` block and use the ``expect`` syntax instead
4. Use the shared example for every ``it`` in the ``describe 'Create'`` block
5. Add an expectation for the error message to the shared example (that is completely missing in the specs, as far as I can see)

Thanks a lot!