activesupport's to_xml shouldn't modify options hash

This patch changes the to_xml core extensions for Array and Hash so that they dup the options hash instead of modifying the passed in version. The current behavior, as demonstrated in script/console, looks like this:

>> options = {:skip_instruct => true}
=> {:skip_instruct=>true}
>> [].to_xml(options)
=> "<nil-classes type=\"array\"/>\n"
>> options
=> {:builder=><nil-classes type="array"/>
<inspect/>
, :indent=>2}
>> 

Notice the options hash has been modified by the to_xml call. With included patch, the options hash is not modified:

>> options = {:skip_instruct => true}
=> {:skip_instruct=>true}
>> [].to_xml(options)
=> "<nil-classes type=\"array\"/>\n"
>> options
=> {:skip_instruct=>true}