Fix for hasFocus / checked binding if not applied via DataBindProvider

[mcselle]

Problem

An observable on hasFocus binding would not fire on focus changes if the binding was not applied via the dataBindProvider (e.g. LegacyBinding)

Testcase

[...]
const createElementWithHasFocusBinding  = {
  init: (element: HTMLElement, valueAccessor: () => any) => {
    let parent = $(element);
    let $hasFocus = valueAccessor();
    
    applyBindingsToNode(parent[0], { hasFocus: $hasFocus })
    }
  };
options.bindingProviderInstance.bindingHandlers.set("customBinding",  createElementWithHasFocusBinding)

testNode.innerHTML = `<input data-bind='customBinding: customProps' />`

var $hasFocus= observable(false)
applyBindings({customProps: $hasFocus}, testNode );
[...]

Fix

//  in hasFocus.ts
[...]
var modelValue = valueAccessor(isFocused, {onlyIfChanged: true});
if (isWriteableObservable(modelValue) && (modelValue.peek() !== isFocused)) {
  modelValue(isFocused);
}
[...]

Found this bug while migrating from ko to tko, we have a custom LegacyBinding that applies bindings itself.
So its not a DataBindProvider which would otherwise trigger Parser.ConvertToAccessor() and therefore override the value function.

// in parser.ts
  convertToAccessors (result, context, globals, node) {
    objectForEach(result, (name, value) => {
      if (value instanceof Identifier) {
        Object.defineProperty(result, name, {
          value: function (optionalValue, options) {
            const currentValue = value.get_value(undefined, context, globals, node)
            if (arguments.length === 0) { return currentValue }
            const unchanged = optionalValue === currentValue
            if (options && options.onlyIfChanged && unchanged) { return }
            return value.set_value(optionalValue, context, globals) // <------------------------ THIS is missing
          }
        [...]

I guess after #202 is merged it would be best to migrate the hasFocus / checked / etc. Bindings like the value Binding which already inherites from BindingHandler that basically contains the same logic as my quick fix

// in BindingHandler.ts
get value () { return this.valueAccessor() }
set value (v) {
  const va = this.valueAccessor()
  if (isWriteableObservable(va)) {
    va(v)
  } else {
    this.valueAccessor(v)
  }
}

[phillipc]

Good work! I know it was not easy to find.