Add reddit example

[hadley]

library(httr)
library(purrr)

app <- oauth_app("reddit", "bvmjj2EOBvOknQ", "n8ueSvTNdlE0BDDJpLljvmgUGUw")
reddit <- oauth_endpoint(
  authorize = "https://www.reddit.com/api/v1/authorize",
  access = "https://www.reddit.com/api/v1/access_token"
)

token <- oauth2.0_token(reddit, app, scope = "read modposts", use_basic_auth = TRUE)

[muschellij2]

This is currently giving

Error in init_oauth2.0(self$endpoint, self$app, scope = self$params$scope,  : 
  Too Many Requests (RFC 6585) (HTTP 429). Failed to get an access token.

I think it's because the User-Agent is using the default from Reddit, which gets throttled quickly. Is it possible to add User-Agent to the header of oauth2.0_token? I can't pass using user_params, even if as_header = TRUE.