fix default value

[cehbrecht]

Overview

The PR changes the initialization of a default value.

• A default value will only be set when min_occurs==0.
• When a default value is configured and min_occurs>=1 a warning will be shown in the log.

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

[cehbrecht]

@fmigneault Please have a look at this PR. When accepted I can backport it also to 4.4.x.

[coveralls]

Coverage remained the same at 0.0% when pulling a063626 on cehbrecht:fix-min-occurs into 584f792 on geopython:main.