improper formatting of dates sent to cbdate and cedate parameters

There is an issue where RAQSAPI is not properly formatting dates passed to the cedate or cbdate parameters. e.g.

aqs_monitors_by_site(parameter = '42401',
                      bdate = as.Date('20150501',
                                      format='%Y%m%d'),
                      edate = as.Date('20190501',
                                      format='%Y%m%d'),
                      stateFIPS = '15',
                      countycode = '001',
                      sitenum = '0007',
                      cbdate= as.Date('20150501',
                                      format='%Y%m%d')

running this code will throw the following error message

Error in `purrr::pmap()` at RAQSAPI/R/bysite.R:66:3:
ℹ In index: 1.
Caused by error in `httr2::req_perform()`:
! HTTP 400 Bad Request.
ℹ At server request time: Wed, 05 Aug 2026 19:51:50 GMT RAQSAPI experienced an error while processing the following url:
  https://aqs.epa.gov/data/api/monitors/bySite?email=<redacted>&key=<redacted>&param=42401&bdate=20150501&edate=20190501&state=15&county=001&site=0007&cbdate=2015-05-01 with status_code: 400 and status message:
  400 Server error message: cbdate: 2015-05-01, requires the following format: yyyymmdd.
Run `rlang::last_trace()` to see where the error occurred.

Start in RAQSAPI/R/bysite.R at the pmap call reported near line 66, and reproduce the provided aqs_monitors_by_site example. Trace how Date values are serialized for cbdate and cedate, then verify that both parameters are sent in the API's yyyymmdd format and the request no longer returns HTTP 400.