Properly escape leaf values in XML responses

Previously "unsafe" data e.g. a URL with an & in it caused Eve to
return a malformed XML response.

Closes #174
Merge branch '174-escape-xml-leaf-values' of https://github.com/kynan/eve into kynan-174-escape-xml-leaf-values
Proper version for versionchanged tag.
Test that XML leaves are being properly escaped.
[fix] Properly escape leaf values in XML responses.
Florian Rathgeber
Merge branch 'kynan-174-escape-xml-leaf-values' into develop
Fix a dangerous default argument, and PEP8s.