Don't escape processing instruction value in the `format` function.

[mkwiatkowski]

With escaping enabled "format 10" test was failing with:

✗ format 10

    { processingInstructions = [{ name = "xml", value = "version="1.0"" }], docType = Nothing, root = Element "a" [] [] }
    ╷
    │ Expect.equal
    ╵
    { processingInstructions = [{ name = "xml", value = "version=\"1.0\"" }], docType = Nothing, root = Element "a" [] [] }

I made the simplest change that would pass all tests. Another approach would be to make ProcessingInstruction.value be a (String, Maybe String) tuple or perhaps a List Attribute, although that would be a breaking change.

[mkwiatkowski]

@jinjor Hi there, any chance of this being merged and released?