Add option to mark a field as null

To remove date fields from Solr when doing Atomic updates, there's need for an option to mark a field as "null" (i.e. delete a field). Currently there does not seem to be a way to do this using Solarium.

For example:
```
<field name="DateField" null="true"></field>
```

Simply leaving the field empty does not work, because it's not accepted as a valid date value.

http://wiki.apache.org/solr/UpdateXmlMessages#Examples_of_adding_docs_with_various_optional_attributes