Escape all ampersands in relationship nodes with '&'

Without this Nokogiri will downcase the attributes for some reason
which I assume must be expected behavior since the issue hasn't been
fixed.
Issue Reference: sparklemotion/nokogiri#961

lib/sablon/document_object_model/relationships.rb

@@ -104,7 +104,7 @@ def next_rid
       # Builds the relationship WordML tag and returns it
       def relationship_tag(rel_attr)
         attr_str = rel_attr.map { |k, v| %(#{k}="#{v}") }.join(' ')
-        "<Relationship #{attr_str}/>"
+        "<Relationship #{attr_str}/>".gsub(/&(?!amp;)/, '&amp;')
       end
     end
   end