Operation.PollUntilCompleted duplicates version header

[jskeet]

We provide a convenience method on Operation<TResult, TMetadata> to automatically poll the operation until it has completed. That method takes a CallSettings as normal, but it has to take account of things in a slightly different way. It needs the effective call settings for GetOperation so it can apply the earlier of "per method call deadline" or "overall deadline" (like an internal retry does) - so we have a special method in OperationsClient of GetEffectiveCallSettingsForGetOperation(CallSettings).

The polling code uses this to obtain a CallSettings, but then passes that to the normal GetOperation method (indirectly), which merges it with the default settings. That means the header mutation action happens twice :( We effectively want to say "use these exact call settings, don't merge them with anything". I think.