Skip null properties

I'm author of http://easywsdl.com. One of our user has a problem that his webservice throws exception when we sent null property as <property nil="true" />. The solution is to basically skip null properties from soap request. 
My change allows to ksoap2 skip properties with null values from the soap request. This is of course configurable (via skipNullProperties field) and the default value is to use the old behavior (so this change shouldn't break any existing code).