Implement a fetch_raw option, only_supports_raw instead of only_supports_classic

[brookslogan]

The endpoints marked with only_supports_classic = TRUE, such as meta, were designed in the original Epidata interface that received JSON and parsed it into a nested list structure. Changing only_supports_classic to only_supports_json or only_supports_classic_and_json prevents some strange print output on the nested data frames (and may allow an easier transition for those using the original Epidata, although there are probably not many/any active human users at this point).

[brookslogan]

fetch_json doesn't actually do what I expected. It doesn't take the json and just convert it to an R object; it tries for a data frame. And fetch_classic also tries to do the same. So some of these old endpoints may not output anything very nice for either fetch_classic or fetch_json. We may need something different for them.

[brookslogan]

Another test case where we want "raw" format rather than "classic":

ec_old = Epidata$delphi("ec", 201501)
ec = delphi("ec","201501") %>% fetch_classic()

It looks like the difference is due to content(res, 'parsed') vs. r <- httr::content(res, "text", encoding = "UTF-8") then m <- jsonlite::fromJSON(r).

[dshemetov]

Closed by #99