XmlSerializer WSDL: XmlAttribute: Set use="required" attribute

[vidrenning]

If a member has been annotated with the [XmlAttribute] attribute, and it is a value type, such as int, it will get the use="required" attribute. However if it has a ShouldSerialize*() method, then it is considered optional and the use="required" attribute is not added.

Added a new test to verify this behavior.

Fixes #1006

[andersjonsson]

Looks good!

Maybe you should check that the return type of the ShouldSerialize*()-methods is System.Boolean.
On the other hand serialization will fail if the return type is something else, so the user that does something like that will have bigger issues than faulty wsdl 😛

[vidrenning]

Thanks for merging! Before implementing this I have tested the behavior of the .NET Framework ASMX Web Service WSDL generator. In that generator, the return type of the ShouldSerialize*() methods is also not checked. The WSDL is the same, no matter if a int ShouldSerialize*() method or a bool ShouldSerialize*() method is defined. Therefore I decided to also not check it, to maximize compatibility with the .NET Framework ASMX Web Service WSDL generator. 😄