Add eof? method to NullIO? (#1169)

* Added eof? method to NullIO


See  https://github.com/rack/rack/blob/master/lib/rack/multipart/parser.rb#L68

lib/puma/null_io.rb

@@ -26,6 +26,10 @@ module Puma
       0
     end
 
+    def eof?
+      true
+    end
+
     def sync=(v)
     end
 

test/test_null_io.rb

@@ -9,6 +9,10 @@ class TestNullIO < Minitest::Test
     self.nio = Puma::NullIO.new
   end
 
+  def test_eof_returns_true
+    assert nio.eof?
+  end
+
   def test_gets_returns_nil
     assert_nil nio.gets
   end