Running HTML responses through `DOMParser#parseFromString` results in
complete `HTMLDocument` instances with unnecessary surrounding tags.
For example:
new DOMParser().parseFromString('<p>hello</p>', 'text/html')
Will output:
<html>
<head></head>
<body>
<p>hello</p>
</body>
</html>
This is passed to the `ajax:success` handler as `event.detail[0]`
(`data`), but cannot be used directly without first traversing the
document.
To resolve this, only XML content is passed through `parseFromString`,
while HTML content is treated as plain-text.
This matches the behavior of jquery-ujs, which relied on jQuery's
response-type inference.
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