Remove redundant branches from SessionManagementConfigurer

[OrangeDog]

I've complained about the style in which these are written before, but this was a prime example of how it over-complicates the control flow, leading to (in this case thankfully not dangerous) errors.

Making use of else and && would simplify this, and other configuration code, even further.
For this case something like this would be best, but I've had changes like this rejected before.

if (this.invalidSessionStrategy == null && this.invalidSessionUrl != null) {
    this.invalidSessionStrategy = new SimpleRedirectInvalidSessionStrategy(
            this.invalidSessionUrl);
}
return this.invalidSessionStrategy;

[jzheaux]

Thanks for the pull request, @OrangeDog. I've left a bit of feedback.

If you rebase, I believe you'll see that failing integration test turn green.

[jzheaux]

@OrangeDog Thanks for looking into this.

Taking a look at getExpiredSessionStrategy below, it looks like there may be an opportunity for cleanup there as well.

What would you think of making these two methods look the same, something like:

if (strategy != null) {
    return strategy;
}

if (url == null) {
    return null;
}

strategy = // get the default strategy
return strategy;

I believe this would get rid of the dead branches as well as keep each if statement simple.

[jzheaux]

Thanks, @OrangeDog, this is now merged into master via ee6df17.

Note that I locally rebased the branch and then updated the commit to comply with our commit formatting guidelines.