Provider helper methods should be able to provide value and attributes

[barmintor]

For instance:
def relation
[ 'value without atribute', [ 'french value', { 'xml:lang' => 'fre' } ] ]
end
gives
dc:relationvalue without atribute</dc:relation><dc:relation xml:lang=fre>french value</dc:relation>

Extracted from #47

[barmintor]

I'm not sure, given the private method that is implied to be overridden here, that this is a safe change - for example, if a single value with attributes were returned as a tuple for value_for, it would be serialized as ... a value with attributes, and an error! This seems like a good use case, but I think the API would have to change.