Fix WebEngineSettings::setDefaultTextEncoding()

Add the missing default_encoding to the web preferences.

Use a 0ms timer instead of the 100ms in order to apply the settings
as soon as possible and avoid waiting mechanisms in the test.

Change-Id: I12f1a6dd68b29c2b287b160eea1ad5a5fb169c14
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>