covidcast's as_of should not "require" an epirange, and does not properly handle epiranges

[brookslogan]

See line here. as_of in the covidcast package allows only a single date; we should support that in epidatr [it looks like we already support this in epidatr, but we should make sure to test we can use a Date class here after #47 is fixed, and/or clarify the error message that an "epirange" is required]. Further, epiranges aren't actually handled properly here; providing a non-zero-width epirange only produces results for one as_of (plus no column for which as_of it came from --- note that as_of and issue are not interchangeable). Maybe actually handling epiranges could be made into a separate "enhancement" request issue; for now, maybe we should just reject epiranges and allow only single dates for as_of.

Currently working around for the single-as-of case like this:

  epidatr::covidcast("hhs", "confirmed_admissions_influenza_1d", "day", geo_type,
                     epidatr::epirange(format(target_date_range[[1L]], "%Y%m%d"),
                                       format(target_date_range[[2L]], "%Y%m%d")),
                     geo_values,
                     as_of=epidatr::epirange(format(evaluation_as_of, "%Y%m%d"),
                                             format(evaluation_as_of, "%Y%m%d"))
                     ) %>%
    epidatr::fetch_tbl()

library(epidatr)
library(magrittr)
generates_misleading_error =
  epidatr::covidcast("hhs", "confirmed_admissions_influenza_1d",
                     "day", "nation",
                     epidatr::epirange("20220801", "20220809"),
                     as_of = as.Date("2022-09-09"),
                     "us") %>%
  fetch_tbl()
#> Error in `check_single_epirange_param()`:
#> ! argument as_of is not a epirange
works_but_could_be_shorter =
  epidatr::covidcast("hhs", "confirmed_admissions_influenza_1d",
                     "day", "nation",
                     epidatr::epirange("20220801", "20220809"),
                     as_of = format(as.Date("2022-09-09"), "%Y%m%d"),
                     "us") %>%
  fetch_tbl()
wrong =
  epidatr::covidcast("hhs", "confirmed_admissions_influenza_1d",
                     "day", "nation",
                     epidatr::epirange("20220801", "20220809"),
                     as_of = epidatr::epirange(
                       format(as.Date("2022-09-09"), "%Y%m%d"),
                       format(as.Date("2022-09-20"), "%Y%m%d")
                     ),
                     "us") %>%
  fetch_tbl()
all.equal(works_but_could_be_shorter, wrong) # TRUE is bad
#> [1] TRUE

^{Created on 2022-09-27 by the reprex package (v2.0.1)}