backport: fix default value (#631)

[cehbrecht]

Overview

This PR backports the fix for default values (#631)

Related Issue / Discussion

625

Additional Information

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• [ ] I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• [x ] I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage remained the same at 0.0% when pulling 9f7dd343615685f18047944d7988f8eab5839d15 on cehbrecht:backport-fix-631 into 2fef4269f71c48621e03cc5b487e9ff24d20920f on geopython:pywps-4.4.

[cehbrecht]

Same as PR #631 without changes.