omit element / xsi:nil="true" elemet control

by Leoš Bitto:

I would follow the principle of least surprise - be as explicit as you
possibly can:

Argument SOAPOmitElement.new doesn't generate any element (not even a
newline)
Argument SOAPNil.new generates xsi:nil="true" element (as nil does now
- not xsi:type=nil)
Argument nil generates xsi:nil="true" element (backwards
compatibility) and a warning if $DEBUG, suggesting using SOAPNil.new

by tpo:

I would let the user explicitly set the beheour, optimally top->down.

I explect that depending on the SOAP Implementation you are talking to
either it will like or won't when you send it empty elements. Thus
being able to tell at the driver level not to send empty elements
would be nice. Thus:

    driver.options[:send_empty_messages]=false

OTOH it might be that you need to tweak /some/ specific part of a sent
message *only*. Then it would be nice to be able to do that as well,
thus:

   element.send_empty_message = false

(In [1810]) * for literal service, let LiteralRegistry? treat nil parameter as same as WSDLLiteralRegistry. omit element if minOccurs == '0' and otherwise add xsi:nil="true". closes #343.