Fix `issue` parsing in `fluview` endpoint

[brookslogan]

It looks like issue is set to be parsed as a date when it's actually an epiweek; the result is NA issues:

  library(delphi.epidata)
  library(magrittr)
  fluview("hhs1", epirange(201501,201503)) %>% fetch_tbl()
#> Warning: One or more parsing issues, see `problems()` for details
#> # A tibble: 3 × 16
#>   release_date region issue  epiweek      lag num_ili num_patients
#>   <chr>        <chr>  <date> <date>     <int>   <int>        <int>
#> 1 2017-10-24   hhs1   NA     2015-01-04   143     915        48019
#> 2 2017-10-24   hhs1   NA     2015-01-11   142    1306        51021
#> 3 2017-10-24   hhs1   NA     2015-01-18   141    1923        50890
#> # … with 9 more variables: num_providers <dbl>, num_age_0 <int>,
#> #   num_age_1 <int>, num_age_2 <int>, num_age_3 <int>, num_age_4 <int>,
#> #   num_age_5 <int>, wili <dbl>, ili <dbl>
  print(readr::problems())
#> # A tibble: 3 × 5
#>     row   col expected         actual file                            
#>   <int> <int> <chr>            <chr>  <chr>                           
#> 1     2     3 date like %Y%m%d 201740 /tmp/Rtmp2ckL6r/file40877e0d5bbf
#> 2     3     3 date like %Y%m%d 201740 /tmp/Rtmp2ckL6r/file40877e0d5bbf
#> 3     4     3 date like %Y%m%d 201740 /tmp/Rtmp2ckL6r/file40877e0d5bbf

^{Created on 2022-07-13 by the reprex package (v2.0.1) --- and fixed up due to readr::problems() somehow being an empty 0x4 tibble without the file column}