Fixed an issue with IOSource#read_until when reaching the end of a file (#288)

Fix GH-287

## Why?
If `@source = nil` is set at the end of the file, prevent
`IOSource#read_until` from raising an error.

> /.gems/gems/rexml-3.4.2/lib/rexml/parsers/baseparser.rb:524:in
'REXML::Parsers::BaseParser#pull_event': #<NoMethodError: undefined
method 'eof?' for nil> (REXML::ParseException)
> .gems/gems/rexml-3.4.2/lib/rexml/source.rb:275:in
'REXML::IOSource#read_until'


https://github.com/ruby/rexml/blob/185bdc737da406ba4f9564726849ad3477858eb2/lib/rexml/source.rb#L266-L276

1. Reaching the end of a file in `IOSource#read`.
2. If the string remains in `@scanner.rest`, use
`@scanner.scan_until(pattern)` to return `str`.
3. If `@source` is `nil`, an exception occurs.

Author: naitoh

lib/rexml/source.rb

@@ -272,7 +272,7 @@ def read_until(term)
         @scanner << readline(term)
       end
       if str
-        read if @scanner.eos? and !@source.eof?
+        read if @scanner.eos? and @source and !@source.eof?
         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