Outstanding issues for migration to jsonlite

[wch]

The development version of Shiny has switched from RJSONIO to jsonlite, but there are some remaining differences in behavior that should be addressed before a release.

• toJSON(auto_unbox=TRUE) can generate invalid JSON: toJSON(auto_unbox=TRUE) can generate invalid JSON jeroen/jsonlite#71. (Workaround already in Shiny)
• toJSON drops names of named vectors: toJSON drops names of named vectors jeroen/jsonlite#76
• toJSON for objects wrapped with I() differs from RJSONIO's behavior: in Shiny JS getting passed as a string timelyportfolio/parcoords#6 (comment). For this one it's not clear to me that we should try to mimic RJSONIO's behavior.