Raise an error for missing namespace prefix

[Zirro]

Test case:

const saxes = require("saxes"),
      parser = new saxes.SaxesParser({ xmlns: true });

parser.onerror = function (e) {
  console.log(e);
};

parser.write('<span xmlns:="urn:x-test:test">12</span>').close();

I believe the code above is expected to produce an error, based on the following test in https://github.com/web-platform-tests/wpt/blob/master/domparsing/DOMParser-parseFromString-xml-parsererror.html#L24. It's currently passing in two browsers.

[lddubeau]

Thank you! 3.1.5 has the fix.