query parameter to signature_v4_auth assumes all queries are strings

[cboettig]

An acceptable query list to httr can include integers, e.g. query = list("max-keys" = 2). This causes signature_v4_auth throws the relatively obscure error Error in strsplit(URL, "") : non-character argument even though httr is capable of handling query lists of mixed type and handles the conversions correctly.

[leeper]

The problem is here. utils::URLencode apparently only accepts a character URL argument. Should be a simple fix to coerce the query string values to character before URL-encoding.