Fix generation of input types

[Pimentoso]

Input types generation is wrong, like also noticed in this issue #12

Input type fields should be like:

argument :id, Integer, required: false

Classes are actually generated like:

field :id, Integer, null: true

This PR should fix that.

[ajsharp]

Apologies on the delay here, not sure how I missed this for so long. Thanks for doing this.