add button content tag to button_to with block, fixes #1772

This PR fixes #1772 respecting application with the old behavior and adding a warning for it.
Removing the deprecation would shrink the code significantly.
No, this is the default value if omitted.
http://www.w3.org/TR/html401/interact/forms.html#h-17.5
http://www.w3.org/TR/html5/forms.html#the-button-element