Description:
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When calling a SOAP function or method in WSDL mode that has optional arguments, the default value for those arguments will be ignored if the SOAP request does not specify those parameters or sets them to nil. Instead, the function will receive NULL for all such parameters.

Test script:
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Here is a fully self-contained test script: http://pastebin.com/szEye5nv

The script writes the WSDL file to the system's temporary directory.

Expected result:
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<?xml version="1.0" encoding="UTF-8"?>
<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope" xmlns:ns1="http://example.com/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:enc="http://www.w3.org/2003/05/soap-encoding"><env:Body xmlns:rpc="http://www.w3.org/2003/05/soap-rpc"><ns1:exampleFunctionResponse env:encodingStyle="http://www.w3.org/2003/05/soap-encoding"><rpc:result>return</rpc:result><return xsi:type="xsd:string">param1 = 'SET', param2 = NULL, param3 = NULL, param4 = NULL</return></ns1:exampleFunctionResponse></env:Body></env:Envelope>

Actual result:
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<?xml version="1.0" encoding="UTF-8"?>
<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope" xmlns:ns1="http://example.com/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:enc="http://www.w3.org/2003/05/soap-encoding"><env:Body xmlns:rpc="http://www.w3.org/2003/05/soap-rpc"><ns1:exampleFunctionResponse env:encodingStyle="http://www.w3.org/2003/05/soap-encoding"><rpc:result>return</rpc:result><return xsi:type="xsd:string">param1 = 'SET', param2 = 'foo', param3 = 'bar', param4 = 'baz'</return></ns1:exampleFunctionResponse></env:Body></env:Envelope>