Use null Instead of false When No Default Value is Provided

[tao-qian]

Code for parseBoolean:

   public static boolean parseBoolean(String s) {
        return ((s != null) && s.equalsIgnoreCase("true"));
    }

So when the BooleanProperty has no default value (e.g. s == null), we are setting the default value as false.

One of the problems caused by this issue:
kongchen/swagger-maven-plugin#284

[fehguy]

@frantuma is this good to merge?

[JoshIsOnIt]

I could really use this as well. Any update on the merge?

[webron]

@thebeeber unfortunately this didn't make the cut for 1.5.10, but we'll try to push it to 1.5.11 (no ETA yet).