Add typed getter for session

[gdaniel]

Session is implemented as a Map<Object, Object>, so we have a single way to access session values: session.get(key), which returns an Object.

Most of the time we know what we are storing in the session though, and we have to cast the result of get:

String myString = (String) context.getSession().get("myKey");

While this is fine for simple types it is annoying for complex ones, especially when accessed in transitions where we typically want a compact syntax:

myState.body(...)
    .next()
    .when(((MyComplexType)context.getSession().get("myKey")).getX() == 2).moveTo(...)

This could be simplified by implementing typed getters for the session:

myState.body(...)
    .next()
    .when(context.getSession().get("myKey", MyComplexType.class).getX() == 2).moveTo(...)

Additional getters could be provided for basic types: getAsString, getAsInt, etc

[gdaniel]

See this piece of code:

private static class MyMap<K, V> extends HashMap<K, V> {

        public String getAsString(K key) {
            return get(key, String.class);
        }

        public Integer getAsInt(K key) {
            return get(key, Integer.class);
        }

        public <T> T get(K key, Class<T> type) {
            Object o = this.get(key);
            return (T) o;
        }

    }