Handle explicit optional parameter after default parameter

[ds5678]

Resolves #3469

Solution

• Any comments on the approach taken, its consistency with surrounding code, etc.
  • I implemented this in an extensions class, which is a little bit removed from the code that uses it.
• Which part of this PR is most in need of attention/improvement?
  • Do you think my extension method needs more documentation?
  • Should there be any additional tests involving decimal constants?
• At least one test covering the code changed

[ds5678]

Is there anything else that needs done for this pull request?

[siegfriedpammer]

Sorry for the wait... I had second thoughts about the use of an extension method (and then other things came up) instead of implementing this directly in the type system, but it's fine, because this way it's independent from the actual implementation.