v2.0.20: Use xml name (#265)

* Update testserver

* Bump to v2.0.19

* Fix generator and update testserver version

* Accept that .XmlName sometimes throws for some reason

* Update autorest

* Fix polymorphism test

* Downgrade autorest testserver version for now

* Update package.json

* Bump to v2.0.20. Fix testserver version range.