Fix: changed geojson mimetype

[huard]

Overview

Closes #528

(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

[coveralls]

Coverage remained the same at 74.26% when pulling d39778b on fix-223 into 6cfd071 on master.

[cehbrecht]

@huard thanks :)