Don't set read-only class if readonly is null

toggleClass() requires a boolean value, not just a truthy/falsy one.
When readonly is null, the current code results in the read-only class
being toggled (since null is treated the same as the argument not being
provided), resutling in indeterminate results depending on what the
previous value of readonly was.

To avoid this surprising behavior, just coerce the value of readonly to
a boolean before passing it to toggleClass(), so that null will always
result in no read-only class, same as if the readonly attribute was not
provided.

addon/mixins/checkbox.js

@@ -20,7 +20,7 @@ var CheckboxMixin = Ember.Mixin.create(Base, {
       settings.onChange = this.get('_onChange');
     }
     if (this._hasOwnProperty(this.attrs, 'readonly') || this.get('readonly') != null) {
-      this.$().toggleClass('read-only', this.get('readonly'));
+      this.$().toggleClass('read-only', Boolean(this.get('readonly')));
     }
   },
 
@@ -83,7 +83,7 @@ var CheckboxMixin = Ember.Mixin.create(Base, {
     // Handle readonly
     if (attrName === 'readonly') {
       // We need to add a class verses updating the property, since semantic is caching the value internall
-      return this.$().toggleClass('read-only', attrValue);
+      return this.$().toggleClass('read-only', Boolean(attrValue));
     }
     // Default
     return this._super(...arguments);