fix error on properties tag, now it returns the property of paragrah

Author: miko53

lib/docx/containers/container.rb

@@ -7,12 +7,12 @@ module Container
         # Relation methods
         # TODO: Create a properties object, include Element
         def properties
-          @node.at_xpath("./#{@properties_tag}")
+          @node.at_xpath("./w:#{@properties_tag}")
         end
 
         # Erase text within an element
         def blank!
-          @node.xpath(".//w:t").each {|t| t.content = '' }
+          @node.xpath('.//w:t').each { |t| t.content = '' }
         end
 
         def remove!