fix #968 with a single argument XPtr constructor

[stephematician]

as<> will no longer modify tags/protected data of an existing external pointer. Modifying the tags/protected data of an external pointer via the XPtr(SEXP,SEXP,SEXP) constructor is made more explicit by removing the two default arguments, and XPtr(SEXP) is implemented separately (as explicit) and does not modify the external pointer's tags/protected data.

Added a self-tag test which passed here. I was unable to test if downstream packages are affected by this change.

[eddelbuettel]

Lovely!

was unable to test if downstream packages are affected

I do and will. Thanks for the PR!

[eddelbuettel]

Looks nice and clean and simple!

[eddelbuettel]

Do you want to drop a short paragraph in ChangeLog, or do you want me to?

[stephematician]

Do you want to drop a short paragraph in ChangeLog, or do you want me to?

I'll give it a go.

[eddelbuettel]

Thanks, If you use The One Editor Everybody Should Use (kidding) it is just C-x 4 a from the file you want to record. Everybody else gets to copy and paste. Two spaces around name and email.

[stephematician]

I'm hoping a PR from a vim user is still acceptable - let me know if any other changes needed.

[stephematician]

Oh no - it added my .swp file :/ Reject!

[eddelbuettel]

That's what you get for using A Very Bad Editor ;-)

[eddelbuettel]

Kidding aside the single worst Unix command known to man is git add in wildcard mode.

Of course, those of us who use "that" editor have the undeescribably awesome Magit mode and have much much finger grained control over the bazillion git commands and subcommands. ....

And please reopen and just a commit to delete the .swp file.