LoadEngine uses mockEngine by default if it's on classpath

[mbussoMeli]

is this behaviour a design rule?. I would like to use the real loader in test scope. For instance, I tried to run a test with kvs in memory but kvs library uses this library which by default load the mockEngine if it's on classpath threfore I couldn't connect to the kvs. Would you accept a PR for adding this feature?

This is the current code that I mention (Engine.java):

@SuppressWarnings("rawtypes")
	private static void loadEngine() {
        for (Builder b : builderLoader) {
            if (b instanceof MockBuilder) {
                builder = b;
                break;
            }
            if (builder != null) throw new IllegalStateException("Many Rest Clients implementations found");
            builder = b;
        }
        loadCallbackFactory();
    }

    @SuppressWarnings("rawtypes")
    private static void loadCallbackFactory() {
        for (CallbackProcessor f : callbackLoader) {
            if (f instanceof MockCallbackProcessor) {
                callbackProcessor = f;
                break;
            }
            if (callbackProcessor != null) throw new IllegalStateException("Many Rest Clients implementations found");

            callbackProcessor = f;
        }
    }