Use === instead of is_a?

For some reason, jruby (in ci only) is failing the new test, rejecting
the assertion that the reopened $stderr is now a File. That's _fair_,
changing the class of a thing after it exists doesn't seem like it
should be possible, and I don't know how it is; it wouldn't surprise me
if jruby _also_ assumes that can't happen somewhere..

spec/rspec/support/spec/stderr_splitter_spec.rb

@@ -103,17 +103,20 @@
   # to do in CaptureStreamToTempfile.
   it 'is able to restore the stream from a cloned StdErrSplitter' do
     cloned = $stderr.clone
-    expect($stderr.to_io).not_to be_a(File)
+    expect(File === $stderr.to_io).to be_falsey
 
     tempfile = Tempfile.new("foo")
+
     begin
       $stderr.reopen(tempfile)
-      expect($stderr.to_io).to be_a(File)
+      expect(File === $stderr.to_io).to be_truthy
+      @checked = true
     ensure
       $stderr.reopen(cloned)
       tempfile.close
       tempfile.unlink
     end
-    expect($stderr.to_io).not_to be_a(File)
+    expect(File === $stderr.to_io).to be_falsey
+    expect(@checked).to be_truthy
   end
 end