virtualize ignores readonly attribute

[Kinrany]

Calling virtualize on a string that has a readonly attribute with no value results in a non-read-only virtual node.

Works as expected with empty value:

virtualize('<input readonly="" />')

Output:

{ 
  sel: 'input',
  data: { attrs: { readonly: '' } },
  children: undefined,
  text: undefined,
  elm: undefined,
  key: undefined 
}

Doesn't work with no value:

virtualize('<input readonly />')

Expected output, same as h('input', { attrs: { readonly: undefined } }):

{ 
  sel: 'input',
- data: { attrs: { readonly: '' } },
+ data: { attrs: { readonly: undefined } },
  children: undefined,
  text: undefined,
  elm: undefined,
  key: undefined 
}

Output:

{ 
  sel: 'input',
- data: { attrs: { readonly: '' } },
+ data: {},
  children: undefined,
  text: undefined,
  elm: undefined,
  key: undefined 
}

Interactive example: https://repl.it/@Kinrany/snabbdom-virtualize-bug

[Kinrany]

Most likely caused by unescapeEntities(text, context) from utils.js.

It expects text to be a string and uses String.prototype.replace.

I guess a simple empty string default parameter would let it handle undefined properly. Not sure if there are other cases, like null.