Remove redundant jsonlite default overrides.

This change keeps the default behavior of `httr` exactly as it is currently, but makes it easier for the user to take advantage of `jsonlite` simplification capabilities. Note that `jsonlite` uses the following defaults:

    fromJSON(txt, simplifyVector = TRUE, simplifyDataFrame = simplifyVector,  simplifyMatrix = simplifyVector, ...)

Hence when using `simplifyVector = FALSE`, then setting `simplifyDataFrame = FALSE, simplifyMatrix = FALSE` is redundant. It is already a bit confusing to the user that `httr` overrides the default behavior of `fromJSON`, but with this pull request the user can get to it using:

    req <- GET("https://api.github.com/users/hadley/orgs")
    data <- content(req, simplifyVector = TRUE)

Which hopefully makes things slightly less confusing then in the current situation, where `simplifyVector = TRUE` does *not* imply `simplifyDataFrame = TRUE` and `simplifyMatrix = TRUE` as it does in `jsonlite`.