Perform strict presence testing on attribute vals

When initializing an Attribute object, the 'vals' option field was being
loosely tested for content presence.  By changing this test to a strict
comparison against undefined, Attribute object creation will be more
consistent across input values.

Fix mcavage/node-ldapjs#178

lib/attribute.js

@@ -76,7 +76,7 @@ function Attribute(options) {
     };
   });
 
-  if (options.vals)
+  if (options.vals !== undefined)
     this.vals = options.vals;
 
 }