Addition of a type argument to method for creating collections in the R driver

This change is related to adding a type argument to the collection interface. In arango_collection.R, the method arango_collection <- function(.database, name, createOnFail=FALSE)... is now: arango_collection <- function(.database, name, createOnFail=FALSE, type=collection_type$DOCUMENT) Please review the updated implementation for details. Earlier, the implementation was always creating a document collection. This is a problem when you need to store edge documents. Mock datafiles related to the change have also been added. Package build and test results are in the attachment.