Add testing and documentation for PKCS#1

[stephen-crawford]

Is your feature request related to a problem?
@cwperks recently corrected an issue were PKCS#1 format was no longer being properly handled: #3289 (review).

While his change corrected the behavior the fact that the issue happened in the first place demonstrates that we have insufficient testing in place to cover support of PKCS#1.

We also lack documentation detailing our support even though people use it and we go out of our way to fix issues related to it.

What solution would you like?
I would like to see additional tests which exercise PKCS#1 support so we can be confident that we offer support. We should also update the documentation to reflect our support.

What alternatives have you considered?
Leaving things how they are does not seem like a good alternative.

[stephen-crawford]

[Triage] Going to mark help wanted.