Support .multi in req_body_form()

[hadley]

Fixes #368

[mgirlich]

I stumbled over this issue

devtools::load_all("~/GitHub/httr2/")
#> ℹ Loading httr2
data <- list(a = c("ab", "c"))

request_test("/post") %>%
  req_body_form(!!!data, .multi = "comma") %>%
  req_verbose(body_req = TRUE) %>% 
  req_perform()
#> -> POST /post HTTP/1.1
#> -> Host: 127.0.0.1:50747
#> -> User-Agent: httr2/0.2.3.9000 r-curl/5.1.0 libcurl/8.1.2
#> -> Accept: */*
#> -> Accept-Encoding: deflate, gzip
#> -> Content-Type: application/x-www-form-urlencoded
#> -> Content-Length: 9
#> -> 
#> >> a=ab,c%20
#> <- HTTP/1.1 200 OK
#> <- Connection: close
#> <- Date: Fri, 10 Nov 2023 10:00:31 GMT
#> <- Content-Type: application/json
#> <- Content-Length: 457
#> <- ETag: "6dd433e5"
#> <-
#> <httr2_response>
#> POST http://127.0.0.1:50747/post
#> Status: 200 OK
#> Content-Type: application/json
#> Body: In memory (457 bytes)

^{Created on 2023-11-10 with reprex v2.0.2}

You can see that the body is a=ab,c%20 instead of a=ab,c because white spaces are appended.
In format_query_param() we need to use justify = "none"

[hadley]

Hmmm, I did that in #355. Maybe that didn't get in this branch? Anyway I'll double check.

[hadley]

No looks like a different problem.

[hadley]

ARGH you need justify = "none" for strings and trim = TRUE for numbers.