Fix XML formatter

[jkbrzt]

Issues

The existing implementation of XML formatting suffers from multiple issues:

• Namespaces get messed up Maven pom.xml files become namespaced when accessed with httpie #389
• Processing instructions get lost XML processing instructions (e.g. xml-stylesheet) lost when formatting #415
• Potentionaly vulnerable Added defusedxml for outputting xml values. #384
• Comments get lost
• Doctype gets lost

Possible Solutions

1. Find/implement a robust XML formatter
2. Remove XML formatting all together Remove XML pretty-print formatting. Fixes #389. #394
3. Only format simple XML without comments, processing instructions, doctype, and namespaces.

Example

Before:

<?pi data?>
<!-- comment -->
<root xmlns='namespace'>
   <element key='value'>text</element>
   <element>text</element>tail
   <empty-element/>
</root>

After:

<ns0:root xmlns:ns0="namespace">
    <ns0:element key="value">text</ns0:element>
    <ns0:element>text</ns0:element>tail
   <ns0:empty-element />
</ns0:root>

[jkbrzt]

Removed the XML formatter for now.