Remove use of mutable as default value in Process.__init__

[gschwind]

Using mutable value as default value for input variable of function
create complex bug because those value will be shared by all Process
instance. Thus remove it and replace it by the safe code.

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(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.
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[coveralls]

Coverage remained the same at 81.378% when pulling 42dfd67 on gschwind:fix-process-init into 7839f52 on geopython:main.