Add option to skip trailing trash in SDMX 2.1 request builder

[eblondel]

From a user:

I keep the package MDstats (https://github.com/zeugner/MDstats) that depends on rsdmx (in a modified form)

Since today, the calling of rsdmx to the ECB API does not work anymore. The reason is that they now throw an error when there is a trailing slash in requests (I use dataflow and datastructure)

For instance

https://data-api.ecb.europa.eu/service/dataflow/all/all/latest

works but

https://data-api.ecb.europa.eu/service/dataflow/all/all/latest/

does not.

The latest rsdmx request builder for the SDMX has those trailing slashes.