fix behavior for deserializing sharedptr to const obj

[pkrenz]

Fix for issue #417

[AzothAmmo]

There are a few issues with your proposed changes, which look good overall.

1. The code needs to compile using C++11 - you are using a few C++14 specific features (e.g. std::decay_t') which we can't support in the current version of cereal.
2. Ideally we want pull requests done against the develop branch, since there are often other changes staged there before being merged into a stable release (master).
3. It would be helpful, but it is not required (I'll do it otherwise) if you could add or modify unit tests to reproduce Deserializing ptr to const objects #417.
4. I'm a bit concerned that the new code creates a temporary shared_ptr before assigning into the wrapper. Perhaps a move assignment would be better here? Previously we were able to create a reference to the actual pointer, but it seems this is not possible if we use a cast.

[WSoptics]

This line appears superfluous to me, @pkrenz ?

[WSoptics]

What I mean is that ptr is reset in the line right after, so there's no point in assigning it a value, is there?