Add breaks if we send None in date, datetime 

Sunburnt breaks if we send date fields set to None. This is because solr only allows only date fields that are proper utc date strings. 

http://stackoverflow.com/questions/7342038/solr-indexing-date-field-with-empty-no-value

To allow it we need to remove all None date fields from update or simply like solrpy all None values form a document.