Fixed an issue with IOSource#read_until when reaching the end of a file

lib/rexml/source.rb

@@ -272,7 +272,7 @@ def read_until(term)
         @scanner << readline(term)
       end
       if str
-        read if @scanner.eos? and [email protected]?
+        read if @scanner.eos? and @source and [email protected]?
         str
       else
         rest = @scanner.rest

test/test_io_source.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: false
+
+require "rexml/source"
+
+module REXMLTests
+  class TestIOSource < Test::Unit::TestCase
+    def setup
+      @source = REXML::SourceFactory.create_from('<?xml version="1.0"?>')
+    end
+
+    sub_test_case("#read_until") do
+      test("eof") do
+        assert_true(@source.read("nonexistent")) # Consume all data
+        assert_false(@source.read("nonexistent")) # Set EOF
+        assert_equal('<?xml version="1.0"?>', @source.read_until(">"))
+      end
+    end
+  end
+end