Fix the mimetype of the raw data output

[gschwind]

Overview

The mime type of the output are not picked from the correct object. This patch fix it.

Related Issue / Discussion

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

[coveralls]

Coverage remained the same at 0.0% when pulling 68625c1 on gschwind:pywps-4.4-003 into 5f486f1 on geopython:pywps-4.4.