Build query string based on the query conditions.
Examples
if (FALSE) { # \dontrun{
mindat_cache_set('page_size',800)
ids<-c("")
hardness_min<- 9.3
fields<- c("name,hardness")
args<- list(ids,hardness_min,fields)
querystring<-mindat_build_querystring(args)
} # }