add includes? check when reading attribute.first

Author: billyyarosh

lib/onelogin/ruby-saml/attributes.rb

@@ -47,7 +47,7 @@ def include?(name)
 
       # Return first value for an attribute
       def single(name)
-        attributes[canonize_name(name)].first if attributes[canonize_name(name)]
+        attributes[canonize_name(name)].first if self.include?(name)
       end
 
       # Return all values for an attribute