code: @SuppressWarnings("unchecked")
  	public <T> RestBuilder addProvider(Class<? extends ContextProvider<T>> provider) {
  
  		Assert.notNull(provider, "Missing context provider!");
  		registeredProviders.put(null, provider);
  		return this;
  	}
docs: Creates a provider that delivers type when needed
  @param provider to be executed when needed
  @param <T> provided object as argument
  @return builder