Add error for POST requests when req.body is undefined

[jrop]

Addresses #128

Sorry it took me so long to create this.

[pdspicer]

I think the check needs to be modified a little bit to potentially avoid "cannot read property 'SAMLResponse' of undefined" errors (i.e. check that req.body exists). In addition I think the check as currently written will break the HTTP-POST binding on SAMLRequests

[markstos]

Also, please add automated test coverage for this. Further, the error message doesn't exactly match the behavior. The error says: "req.body undefined for POST request", but the check is not for this, but for req.body.SAMLResponse. It's possible req.body.SAMLResponse could be undefined, while req.body is still defined.

[cjbarth]

@jrop We will be releasing v3.x in the upcoming weeks. If you would like this included in that release, please address the latest comments and we'll get this landed.

[jrop]

@cjbarth I've since moved on from the job where I needed this, and will not get around to modifying this PR to address the comments. 😢