merge struct fields of type interface{} that contain data with the same concrete type

[bionoren]

If a struct has two fields that are of interface type but contain data that is the same type, we should be able to deepMerge into that. This patch adds test coverage and support for this use case.

What this does, really, is

1. move the dst.Kind() != ptr && isAssignable check to the end (because it took precedence over the "else deepMerge" case)
2. only call deepMerge if the underlying Elem() values are the same kind
3. modify the deepMerge call so that dst is populated (otherwise, if the value being passed to deepMerge has "CanSet() == false", which it does, then the result is lost).

[darccio]

"Reopening" this as PR #105 introduced too many bugs, and this issue isn't fixed with the current state of master.

[darccio]

@bionoren Sorry for the inconvenience, but I'm not able to apply your solution to the current implementation (it is not longer mergeable as PR #105 modified too many lines). If you can open a new PR, I'll be happy to merge as soon as it is ready.