title: React Native change state leads to mobx-state-tree exception
body: <p>I have a problem with changing the value in my state. I have</p>
  <pre><code>state = {
      ......,
      tags: []
  }
  </code></pre>
  <p>and these tags are filled like:</p>
  <pre><code>tags: this.props.userStore.details.tags
  </code></pre>
  <p>tags are a list of dictionaries:</p>
  <pre><code>tags = [{'name': tag1, 'userSelect': true}, {'name': tag2, 'userSelect': false}]
  </code></pre>
  <p>I display them all in the checkbox:</p>
  <pre><code>renderTagCheckbox() {
      return this.state.tags.map((item, key) =&gt; {
        return(
          &lt;TouchableOpacity key={key} style={{flexDirection: &quot;row&quot;, marginTop: 5, justifyContent: &quot;center&quot;}}&gt;
            &lt;CheckBox
              isChecked={item.userSelect}
              onClick={() =&gt; this.changeCheckboxValue(item)}
            /&gt;
            &lt;Text&gt;{item.name}&lt;/Text&gt;
          &lt;/TouchableOpacity&gt;
        )
      })
    }
  </code></pre>
  <p>in my <code>changCheckboxValue()</code> I have:</p>
  <pre><code>  changeCheckboxValue(item) {
      //this.refersSubscriptions.forEach(s =&gt; s.remove())
      let currentTags = this.state.tags
      currentTags.forEach(tag =&gt; {
        if (tag===item) {
          tag.userSelect = !tag.userSelect
        }
      })
    }
  </code></pre>
  <p>and this way I was thinking I could setState with <code>currentTags</code> where the value will be change, but I get an error:</p>
  <p><code>[mobx-state-tree]Cannot modify 'tags@/userStore/details/tags/0', the object is protected and can only be modified by using an action.</code></p>
  <p>Could somebody give me any tips on what am I doing wrong?</p>