QuickJS: fixed ngx_qjs_string().

[hongzhidao]

Fix the function to properly handler strings like "\0" which happens in tests. In js_fetch_objects.t:

for (var i = 0; i < 128; i++) {
  var c = String.fromCodePoint(i);

Refactored out from qjs fetch patches.

[xeioex]

Suggested commit log:

QuickJS: fixed ngx_qjs_string() to handle strings containing "\0".

Otherwise Looks good.