Fix kvp decoding to follow specification

[gschwind]

Overview

In the specification of WPS 1.0.0 the specification state in section 10.2.2.1:

6. All field values and attribute values shall be encoded using the standard Internet
   practice for encoding URLs [IETF RFC 1738].

Thus unquote values and attribute values as expected.

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 0823c32 on gschwind:pywps-4.4-001 into 5f486f1 on geopython:pywps-4.4.