Fixing unit test to accept text/xml; charset=*

[gschwind]

The unit test is to restrictive and do not accept the valid content type
text/xml; charset=utf-8. This patch correct it

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 increased (+0.01%) to 73.074% when pulling 5ec2ec1 on gschwind:fix-003 into c9d46d6 on geopython:master.

[cehbrecht]

Looks reasonable and works for me.

[cehbrecht]

PR looks simple enough ... so I merged it. @gschwind thanks.