refactor collapsing elements

[wibeasley]

There are several places in several functions that repeat code like this.

  if( (length(fields_collapsed)==0L) | is.null(fields_collapsed) | all(nchar(fields_collapsed)==0L) )
    fields_collapsed <- ifelse(is.null(fields), "", paste0(fields, collapse=",")) #This is an empty string if `fields` is NULL.

Pull it out into a reusable & testable function.

• redcap_read()
• redcap_read_oneshot()
• redcap_read_oneshot_eav()
• redcap_metadata_read()