Binary and Ranking modes double ID attribute in poll submit button

Setting the poll's submit ID using th #attributes property results in two IDs, which breaks (X)HTML validation. I believe the #id form property is automatically set by the form API if it isn't already specified, at least for submit buttons, and the form API seems to consider an ID set via #attributes as unset, at least before setting its own and then adding the one from #attributes. Instead, we should probably use #id when we want to explicitly set the ID attribute.

Patch for both binary and ranking mode files is attached.