Support assertEqualXML

[kingcrunch]

It would be nice to easily compare encoded objects like XML or JSON

assert.EqualXMLs(t, "<a foo=\"bar\" x=\"y\" />", "<a x=\"y\"  foo=\"bar\"></a>") // true

[zricethezav]

Looks like JSON is covered: https://github.com/stretchr/testify/blob/master/assert/assertions.go#L1128

[ernesto-jimenez]

Renaming issue since JSON is covered.

XML is a lot trickier to implement. How would you expect it to work? Especially given XML namespaces, canonicalization, etc.

[wregis]

I think it should compare the element comparing name, namespace, attributes and text content and children for quantity and all previous properties.

It might be difficult to test children as there is not a guaranteed stable order for them.

The test is not to ensure namespace compliance.