Can't use `as_of` or `issues` with `Date` vector (any length)

[brookslogan]

library(epidatr)
set_cache(days = 28L, max_size = 2e3)
pub_covidcast("hhs", "confirmed_7dav_incidence_prop", "state", "day", "*", "*", as_of=Sys.Date())
#> Error in charToDate(x): character string is not in a standard unambiguous format

^{Created on 2023-10-02 with reprex v2.0.2}

This appears to be because of this comparison: epidata_call$params$as_of != "*" which appears to attempt date conversion on "*". Note we don't even support as_of="*" in the API, which would be extremely large (returning full snapshots for every version), only issues="*".

[brookslogan]

Same thing happens when specifying a single date in issues =, and some other error message or warning + bad behavior may happen with issues = date vector.

Maybe these checks should be done with identical.

[brookslogan]

This is actually blocking use of as_of and issues when using a Date vector (of any length), regardless of whether we are using caching or not. (length 1 should be supported by both. length > 1 should be supported for issues but not as_of.)