support <nil> as struct member value
support <nil> as struct member value

Although <nil> values are an XML-RPC extension it is widely supported.
This change allows for encoding <nil> values in structs by checking
dictionary values for an instance of NSNull.

lksoft/xmlrpc,corristo/xmlrpc,lksoft/xmlrpc,evolvIQ/cocoa-rpc,lksoft/xmlrpc,evolvIQ/cocoa-rpc,evolvIQ/cocoa-rpc,corristo/xmlrpc,younata/xmlrpc,younata/xmlrpc