Check GML validator

# Description

On mode 1 and 2, GML validator does not seem to be working for this client request (based on OWSLib)

```
import owslib.wps
import collections

url = "http://127.0.0.1:5000/wps"
wps = owslib.wps.WebProcessingService(url, verbose=False, skip_caps=True)
wps.getcapabilities()

process_name = "feature_count"
inputs = [("layer", owslib.wps.GMLMultiPolygonFeatureCollection([[(-102.8184, 39.5273),
(-102.8184, 37.418),
(-101.2363, 37.418),
(-101.2363, 39.5273),
(-102.8184, 39.5273)]]))]

execution = wps.execute(process_name, inputs)

for output in execution.processOutputs:
print("#", output, owslib.wps.printInputOutput(output))
```

store as `request.py` and run using `python3 request.py`

tested on pywps-demo feature_count process

# Environment

- operating system: Linux
- Python version: 3.6
- PyWPS version: master (4.x)
- source/distribution
- [x] git clone
- [ ] Debian
- [ ] PyPI
- [ ] zip/tar.gz
- [ ] other (please specify):
- web server
- [ ] Apache/mod_wsgi
- [ ] CGI
- [x] other (please specify): flask

# Steps to Reproduce

# Additional Information