Fix URI#join for path-rootless, which goes away when merged with a relative path.

Author: gkellogg

lib/rdf/model/uri.rb

@@ -431,7 +431,9 @@ def join(*uris)
           joined_parts[:query] = uri.query
         else
           # Merge path segments from section 5.2.3
-          base_path = path.to_s.sub(/\/[^\/]*$/, '/')
+          # Note that if the path includes no segments, the entire path is removed
+          #  > return a string consisting of the reference's path component appended to all but the last segment of the base URI's path (i.e., excluding any characters after the right-most "/" in the base URI path, or excluding the entire base URI path if it does not contain any "/" characters).
+          base_path = path.to_s.include?('/') ? path.to_s.sub(/\/[^\/]*$/, '/') : ''
           joined_parts[:path] = self.class.normalize_path(base_path + uri.path)
           joined_parts[:query] = uri.query
         end

spec/model_uri_spec.rb

@@ -606,6 +606,8 @@
       %w(http://a/bb/ccc/.. g#s) => "<http://a/bb/ccc/g#s>",
 
       %w(file:///a/bb/ccc/d;p?q g) => "<file:///a/bb/ccc/g>",
+      # merging rootless base URL paths (json-ld-api 397f48b959c4517fef55a7b2623ad432e923240c)
+      %w(tag:example a) => "<tag:a>",
     }.each_pair do |(lhs, rhs), result|
       it "creates #{result} from <#{lhs}> and '#{rhs}'" do
         expect(RDF::URI.new(lhs).join(rhs.to_s).to_base).to eq result