Fix uri_parser read_preference validation

[craigahobbs]

uri_parser fails if there is a read_preference option. This fixes that.

[rozza]

Thanks Craig - I've added https://jira.mongodb.org/browse/PYTHON-471 for this pull request for review.

[behackett]

Hi Craig,

Thanks for this pull request. Using read preferences with the MongoDB URI seems totally broken currently. :(
The only thing this is missing is support for 'readPreference' which is what the URI option is really called. If you want to add that (it's just adding another entry to common.VALIDATORS) great. Otherwise I can do it after merging this.

[behackett]

Perfect. Thanks again for the patch. Merged just in time for 2.4.2.