fix(checkbox): add 'type="button"' to <button> tag

Add attribute 'type="button"' to <button> tag. The missing attribute caused forms with

ion-checkboxes to not be submitted as a normal form

Author: rtomchinsky

ionic/components/checkbox/checkbox.ts

@@ -54,6 +54,7 @@ const CHECKBOX_VALUE_ACCESSOR = new Provider(
       '<div class="checkbox-inner"></div>' +
     '</div>' +
     '<button role="checkbox" ' +
+            'type="button" ' +
             '[id]="id" ' +
             '[attr.aria-checked]="_checked" ' +
             '[attr.aria-labelledby]="_labelId" ' +