Add test for relative IRI starting with a number.

[davidlehn]

jsonld.js had a bug not being strict enough when parsing IRIs. For input like 123:abc it would think 123 was a URI scheme.
digitalbazaar/jsonld.js#523

This may not be a common implementation problem. Perhaps a one-line test case update can be added just in case. Which of the many toRdf 3986 test files would be best for this? A one-liner to ensure 123:abc is considered relative vs absolute would catch the issue.