Plans returning constant(true) throw a step mis-match error
### Summary

On updating grafast from 0.1.1-beta.0 to 0.1.1-beta.3, my plans that return constant(true) throw the following error: `Step mis-match: expected __ValueStep, but instead found ConstantStep`.

### Steps to reproduce

Here's a gist of plugin that throws the error: https://gist.github.com/benjie/f9381378b4909df5ba6df530ba6046d3

### Expected results

In the gist, I expect the plans to return the root query type (note operationPlan().rootValueStep can do this as well).

### Actual results

The error in the summary gets thrown.
