Escape XML responses

# Description

XML responses should be escaped, in order to expose valid XML.

For example, in a service that uses `pywps` package, we currently get this response when querying `?request=GetCapabilities&service=WPS`:

```
...
Maximum number of consecutive frost days (Tn < 0).
...
```

To be valid, the response should be:

```
...
Maximum number of consecutive frost days (Tn < 0).
...
```