Upgrade to beat the rule of solr ...

Example of "add" with optional update attribute to set a field to null (i.e. delete a field):


```xml
<add>
  <doc>
    <field name="employeeId">05991</field>
    <field name="skills" update="set" null="true" />
  </doc>
</add>
```

http://wiki.apache.org/solr/UpdateXmlMessages#Optional_attributes_for_.22field.22