2.15.0 Stable release

[plq]

Here's what needs to be done:

1. [ ] merge #696
2. [ ] have a look at other recent patches that got in,
3. [ ] run the whole test suite one last time, fix stuff that comes up
4. [ ] release it.

Others can help by reviewing code for 1 and 2, provide build logs/fixes for 3.

release can only be done by me but quite simple once everything else is done.

[plq]

Here's the link to the latest jenkins run: https://jenkins.arskom.com.tr/job/spyne/job/origin-stable/214/