trusted domains shall not be ignored on autosetup

We have the mechanism to automatically run setup, when sufficient information is provided in config/autoconfig.php. A $trusted_domains entry however is ignored.

How to test? 
1. Create an autoconfig.php just like a config.php  with specified details on dbtype, dbhost, directory, adminlogin, adminpass, dbuser, dbpass, dbname, trusted_domains. Make sure that the trusted_domains array keeps more information than just the domain/IP under which you will reach your ownCloud :) 
2. Visit the ownCloud web interface, this triggers the autosetup
3.  Check the created config.php. Before applying this, the trusted_domains consists only of the domain you accessed ownCloud with in step 2. After applying this, it trusted_domains will appear as expected.

Needed for UCS, but it is not urgent.

I added the is_array check after testing, i.e. I do another test tomorrow, but it should be fine.

please review @LukasReschke @DeepDiver1975 @PVince81