Add spec for Socket#read_nonblock with a provided buffer

When provided a buffer MRI preserves the original encoding
but TruffleRuby sets the encoding to `Encoding::BINARY`

library/socket/basicsocket/read_nonblock_spec.rb

@@ -18,7 +18,34 @@
 
     it "receives data after it's ready" do
       IO.select([@r], nil, nil, 2)
-      @r.recv_nonblock(5).should == "aaa"
+      @r.read_nonblock(5).should == "aaa"
+    end
+
+    it 'returned data is binary encoded regardless of the external encoding' do
+      IO.select([@r], nil, nil, 2)
+      @r.read_nonblock(1).encoding.should == Encoding::BINARY
+
+      @w.send("aaa", 0, @r.getsockname)
+      @r.set_encoding(Encoding::UTF_8)
+      IO.select([@r], nil, nil, 2)
+      @r.read_nonblock(1).encoding.should == Encoding::BINARY
+    end
+
+    it 'replaces the content of the provided buffer without changing its encoding' do
+      buffer = +"initial data"
+      initial_encoding = buffer.encoding
+
+      IO.select([@r], nil, nil, 2)
+      @r.read_nonblock(5, buffer)
+      buffer.should == "aaa"
+      buffer.encoding.should == initial_encoding
+
+      @w.send("aaa", 0, @r.getsockname)
+      @r.set_encoding(Encoding::ISO_8859_1)
+      IO.select([@r], nil, nil, 2)
+      @r.read_nonblock(5, buffer)
+      buffer.should == "aaa"
+      buffer.encoding.should == initial_encoding
     end
 
     platform_is :linux do