title: Checkbox in rails has three values which is causing issues
body: <p>Hi so i'm new to rails so forgive me if anything sounds daft here.</p>
  <p>Context:
  So i have this situation where we have a fairly important form that we cant default answers for users as it revolves around government tax authorisation related things so our boolean answers essentially have three states,</p>
  <pre><code>nil -&gt; user hasnt provided an answer
  true -&gt; User has said true
  false -&gt; User has said false
  </code></pre>
  <p>The issue im having in our rails backend with this is by default the form.check_box :some_proper is defaulting the input to false when if the value is nil it needs to remain nil.</p>
  <p>So whats happening is a whole series of nil values are changing to false when our form is submitted.</p>
  <p>I have provided some code examples to hopefully lend a hand but I cant find anyway of doing whats needed and to be honest we may just need to swap from checkboxes to select fields or something.</p>
  <pre><code>%dt= form.label :knowledge_intensive
  %dd= form.check_box(:knowledge_intensive, {}, &quot;1&quot;, &quot;0&quot;)
  </code></pre>
  <p>I have attached a gif to illustrate the issue clearer</p>
  <p><a href="https://www.loom.com/embed/6d3c5997e6a841b7b8e86b5ad49147d3" rel="nofollow noreferrer">Demo of issue in GIF Form</a></p>