Allow desc or description to be used for parameter descriptions

I was surprised that you had to use `desc` to describe parameters. Using `description` would throw an `unknown validator` error as Grape interpreted it as a custom validation (ie- `regexp`, etc.).

I added a spec that setting either `desc` or `description` **works** (returns a 200, currently this will crash if you use `description`). 

Let me know if that's an odd spec or if you'd like something in addition. Was a bit unsure where such a seemingly-simple spec should go. Thanks! great gem btw :smile: