Fixes 6716: ServiceInterfaceStrategy fails to create with interface containing multiple methods without parameters

Hi all,

This pull requeste is related to:
https://issues.apache.org/jira/browse/CAMEL-6716

I've added a second method without parameter in the customerservice.wsdl and add an assert in the **testServiceInterfaceStrategyWithClient** method of **ServiceInterfaceStrategyTest** Test class.

Hope this should be useful and it will be merged.

P.S.: I've closed the other pull request and submitted this new one.