Fix button_to's params option to support nested names.

In e6e0579defcfcf94ef1c4c1c7659f374a5335cdb the `params` option was added to the `button_to` helper. However, the patch doesn't support nested hashes so `{a: {b: 'c'}}` for example gets turned into a hidden form input with the name 'a' and the value being the string representation of the `{b: 'c'}` nested hash.

Since Rails supports nested hashes everywhere else (and even in the URL params of link_to and button_to), I believe this to be a bug/unfinished feature.