fix: headers not being properly applied to R httr snippets

[erunion]

🚥 Fix RM-4400

🧰 Changes

This fixes a problem with R httr snippets where headers are being improperly added to snippets:

library(httr)

url <- "https://dash.readme.com/api/v1/api-specification"

queryString <- list(
  perPage = "10"
  page = "1",
)

response <- VERB("GET", url, add_headers(x_readme-version = 'v3.0Authorization = 'Basic REDACTED'), query = queryString, content_type("application/octet-stream"), accept("application/json"))

content(response, "text")

• Multiple headers aren't being separated with a comma
• Header values aren't always being encased with a trailing apostrophe.
• x- headers are being rewritten to x_.

🧬 QA & Testing

I tried getting a Docker integration up and running for R but ran into a bunch of issues with getting the httr package installed. I was able to get it running locally but running the whole suite there's a lot of other problems with R snippets (notably multipart/form-data) that I don't have the energy to address in this PR.

For now, here's what running the headers fixture looks like. Note that the x- headers are being properly sent and returned through the response from https://httpbin.org/headers: