Custom SOAPFault detail unmarshaling

[lizapolyudova]

#180

Background

This PR fixes the described discrepancy between SOAP 1.1 spec and gowsdl implementation of fault detail.

Overview of changes

Add a new function

func (s *Client) 
CallWithFault(soapAction string, request, response, faultDetail interface{}) error

which allows to inject a custom fault detail type, similar to request and response.
Previously existing Call function remain backwards compatible and passes in nil for the fault, which is correctly handled (ignores detail information).

A slight awkwardness with this approach is: you cannot simply rely on Body.Fault == nil as a condition indicating there is no fault, since Fault is initialized with the custom detail type. Instead, there's a boolean faultOccurred, which is filled out during Body unmarshalling.

SOAPFault has to still be a pointer, since it has to be nil to pass a check in TestClient_MTOM, since it enforces that there are not multiple elements inside SOAP body.

[c4milo]

@lizapolyudova, thank you! I will review it after the holidays. 🙇‍♂️

[lizapolyudova]

@c4milo sorry to ping you again. We are running into some issues on our side with not being able to distinguish errors from warnings (which are encoded within the detail of the XML fault) and it's been somewhat frustrating.
lmk if there's anything I can help with.

[c4milo]

@lizapolyudova, apologies for the delay 😓thanks for the contribution! 🙌🏻