Add tests for IO#read_nonblock following IO#ungetc with character convertion en/dis-abled

Author: javanthropus

core/io/read_nonblock_spec.rb

@@ -55,14 +55,25 @@
     @read.read_nonblock(4).should == "hell"
   end
 
-  platform_is_not :windows do # https://bugs.ruby-lang.org/issues/18881
-    it "reads after ungetc with data in the buffer" do
-      @write.write("foobar")
-      c = @read.getc
-      @read.ungetc(c)
-      @read.read_nonblock(3).should == "foo"
-      @read.read_nonblock(3).should == "bar"
-    end
+  it "reads after ungetc with data in the buffer" do
+    @write.write("foobar")
+    @read.set_encoding(
+      'utf-8', universal_newline: false
+    )
+    c = @read.getc
+    @read.ungetc(c)
+    @read.read_nonblock(3).should == "foo"
+    @read.read_nonblock(3).should == "bar"
+  end
+
+  it "raises an exception after ungetc with data in the buffer and character conversion enabled" do
+    @write.write("foobar")
+    @read.set_encoding(
+      'utf-8', universal_newline: true
+    )
+    c = @read.getc
+    @read.ungetc(c)
+    -> { @read.read_nonblock(3).should == "foo" }.should raise_error(IOError)
   end
 
   it "returns less data if that is all that is available" do