Authorize Null parameter in Accessor method

Hi, 

First of all, I thank you for providing us this great driver ;)
I'm updating my source code to use the Accessor feature provided in the datastax driver.

With the current version of the datastax driver, we can't insert null as parameter value of an accessor method. It may be the expected behaviour but from my point of view, a null value should be authorized in the same way as a statement authorized it. 

This patch allows to insert "null" as value for an accessor method parameter.

Maybe this patch is  too simple. Additional controls could be done using the @Param annotation in order to specifying that some parameter are nullable. 

What is your opinion about that ?

Best regards,
Eric