R/Module.R: avoid calling as.character on a C++Object

[bgoodri]

This fixes a mysterious race on Debian unstable with R 3.3.0 beta
that was triggered when loading the rstanarm package

Closes #458

[eddelbuettel]

Thanks so much for submitting the PR.

I'll try to get some feedback from those more knowledgeable than me about Reference Classes / S4, but will also give it the full test treatment. Will some luck this should be in a 0.12.5 release around the time R 3.3.0 comes out.

[eddelbuettel]

Full reverse dependency check passed: 626 packages with no issues, 7 with various issues:

• two with what looks like internal issues in testing: contoureR, kerpg
• two which were lacking rgdal, and I usually skip those: mapview, stplanr
• one of mine/ours which needs an update (also local testing issue): Rblpapi
• one with a C++ build error against Modules -- which may be related to @romainfrancois recent change as I don't think we had other changes there lately: diversitree
• one with an issue against RcppParallel: text2vec so I should talk to @kevinushey .

So I merge this now. We can probably clean the other two easily enough.