Step autocomplete in feature files leads to duplicated text in the step

[gabeseltzer]

Describe the bug (required):

Accepting an autocomplete suggestion in a feature file will give you duplicated code in your step.

To Reproduce (required):

1. Starting with the simple integration test workspace,
2. Add a new step to any scenario and start typing When we
3. The autocomplete will correctly pop up with we implement a ? test
4. If you press enter to accept the autocomplete suggestion
5. Then you end up with the step: When weWhen we implement a ? tests <<< BAD, this still has start of the step that you typed. It should have been replaced by the autocomplete.

Operating system (required):
Windows Subsystem for Linux Ubuntu

Directory structure (optional, but often relevant):
Simple workspace in the integration tests

Pull request with proposed fix (optional):
If you have created a pull request in a fork to fix the issue, please link it here.