Fixed #21606 -- OSMWidget overrides map position passed to widget constructor

I've found a bug in OSMWidget implementation, which I've posted to the Trac. This pull request is a fix for that bug, with tests showing the issue.

Trac link: https://code.djangoproject.com/ticket/21606

In the last commit, I've also enabled running certain tests, even when spatial database is not present, because some tests don't need it (mine for example).

Tested with default settings: 
```
python test/runtests.py --settings=test_sqlite django.contrib.gis
```