Fix: Register WorkflowHandle for gob encoding automatically

[vr-varad]

Fixes #151

[vr-varad]

The workflow handlers registered are:

• baseworkflowhandle
• workflowhandle
• workflowPollingHandle

Now I need to make changes in the tests? // @maxdml

[maxdml]

Thanks @vr-varad , yes we should have a test (either modifying an existing one or adding a new one) where a workflow and a step would return a WorkflowHandle interface.

I don't think we need to register baseworkflowhandle : the user will always get a workflowhandle or workflowPollingHandle.

[maxdml]

@vr-varad also check #166: adding a safeGobRegister helper, which we should use in this PR too.

[vr-varad]

Does the failing test mean the registration has failed? // @maxdml

[maxdml]

Does the failing test mean the registration has failed? // @maxdml

I think there is another, unrelated bug. It looks like we don't check if the concrete value provided to safeGobRegister is nil. Gob does panic if that's the case. I'll be providing a fix soon.

[maxdml]

@vr-varad the issue is that we can't automatically register interfaces during RegisterWorkflow. In this case, when you try to register a workflow that returns an interface, behind the hood we create a zero value and attempt to register it. For all zero values, this works fine, but for interfaces, gob requires that the underlying concrete type has been registered first.

There is another issue I now realize: the workflow handles don't export any field, which makes them unsuitable for encoding as well.

In this case let's just cancel this feature, ignore registration of nil interfaces, and ask users to manually register their types.

[vr-varad]

Makes Sense @maxdml!!