Unexported interface methods are internal

[MichaelUrman]

When an interface method is not exported, its implementation can be assumed to wrap errors. This can be implemented by requiring sel.Sel.IsExported() before checking for isInterface(pass, sel).

Then e.g., the test interface_on_struct can be extended to define a decode(v interface{}) error and call it without issuing an error.

[tomarrell]

Fair enough, if you would like to open a PR and include a test case I'd be happy to review it.