Support non-String attributes

[atjn]

Hello and thank you for making these tools :)

If you add a non-String attribute to any node and ask dom-serializer to parse it, it will treat the attribute value as a string and quickly throw an error.
Here is a condensed example: https://runkit.com/atjn/64073ea61e340b000802a5cd

In my case, I am trying to add a width and height to an SVG node. I know that these values are technically speaking strings, but they are supposed to be parsed as numbers, so I think it makes sense that I can set them to a Number without breaking the parser. This is also how the normal DOM APIs work in JS.

Therefore I would suggest adding a quick conversion to string before encoding the attributes.

If you don't think the parser should support non-string values, then I would suggest adding a check that throws a more concise error. The current one is very hard to decipher:

file:///[...]/node_modules/entities/lib/esm/escape.js:46
    return ret + str.substr(lastIdx);
                     ^

TypeError: str.substr is not a function
    at encodeXML (file:///[...]/node_modules/entities/lib/esm/escape.js:46:22)