Create optgroups in options_for_select

This is a small patch to enable options_for_select to create optgroups from nested arrays. For example:

   options_for_select([ ["First Group", [["Option 1", 1], ["Option 2", 2]]], ["Bare Option", 3] ])

yields (formatted for readability):

<optgroup label="First Group">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
</optgroup>
<option value="3">Bare Option</option>

Note that the nested things don't have to be arrays; the group label is taken from .first of the element, and the .last of the element can be anything that options_for_select supports.

All the other options work exactly as expected (selected, disabled, etc).

Note that this is slightly more general than grouped_options_for_select, as it can produce mixed sets with both bare options and optgroups.

Test included in the patch.