Added :path option to ActiveFedora::QualifiedDublinCoreDatastream#field ...

...in order to support, for example,

mapping the term :dc_type to <dcterms:type> (:type is excluded from ActiveFedora::QualifiedDublinCoreDatastream::DCTERMS).
Implementation simply adds :path (even if nil) to OM::XML::Term.new since initializer falls back to field name anyway.