ChemSpider no longer supports validating inchikey without API key

[stitam]

I noticed that is.inchikey_cs() is not working. I found that is.inchikey_cs() uses the public (i.e. no need for an API key) base url "http://www.chemspider.com/InChI.asmx/IsValidInChIKey?" for querying whether a character string is a valid inchikey and it seems this service has been removed by ChemSpider. I looked at the current ChemSpider APIs and I see that the tool now runs under "https://api.rsc.org/compounds/v1/tools/validate/inchikey" and requires an API key. I have started to implement the fix.