Encoding change for JSON bodies

Between 0.4 and 0.5, something has changed about the way JSON bodies get encoded -- a list like 

    ls <- list(f = "hi") 

was previously encoded as 

    {"f": "hi"} 

but is now being encoded as 

    {"f": ["hi"]}

this of course wreaks havoc on any API that doesn't differentiate a string and a list with one string. in particular, it's causing failures in `bigrquery` as discovered by @deflaux.

i'm still digging to figure out whether the change was on the `httr` side or the `jsonlite` side.
Can you make that `Content-Type` - trying to be consistent internally!