[bug] All buttons will submit the form

[SanderVerkuil]

When having the datepicker inside a form, the buttons to click on the 'next' and 'previous' month also trigger the 'submit' event on the form.
According to the W3C spec the default type of a button is 'submit', so all buttons that don't have a type will submit the parent form.
This issue can be resolved by having a default type value of 'button'.