Fix electric-indent-mode

electric-indent-mode is broken.

electric-indent-mode (electric.el) initially defines electric-indent-chars as follows:

```elisp
(defvar electric-indent-chars '(?\n)
  "Characters that should cause automatic reindentation.")
```
js-mode (js.el) adds more characters like this:

```elisp
(set (make-local-variable 'electric-indent-chars)
     (append "{}():;," electric-indent-chars)) ;FIXME: js2-mode adds "[]*".
```

The resulting electric-indent-chars for buffers where js-mode is active:
```
electric-indent-chars is a variable defined in `electric.el'.
Its value is (123 125 40 41 58 59 44 10)
Local in buffer foo.js; global value is (10)
```
`M-x describe-variable` displays the values in ASCII decimal, not exactly sure why.

js2-mode however, appends to electric-indent-chars incorrectly. Loading js2-mode and M-x describe-value electric-indent-chars gives:
```
electric-indent-chars is a variable defined in `electric.el'.
Its value is ("{" "}" "(" ")" "[" "]" ":" ";" "," "*" 10)
Local in buffer foo.js; global value is (10)
```
This doesn't work properly. None of the string values have any effect. E.g. typing "}" should trigger a reindentation, but it doesn't. Only \n (10) works, which is the inital value set by electric.el.

This PR should fix it by appending to the var the same as js-mode does.