Core htmx internals Tests

• set header works with non-ASCII values1ms ‣

  var xhr = new XMLHttpRequest()
  xhr.open('GET', '/dummy')
  htmx._('safelySetHeaderValue')(xhr, 'Example', 'привет')
  // unfortunately I can't test the value :/
  // https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest