update geojson mimetype in validators to match that of FORMATS

[huard]

Overview

I modified the GeoJSON mimetype in FORMATS but forgot to change it as well in the mapping from mimetypes to validators.

Related Issue / Discussion

#603

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.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines