Write EOF indicated for plain encoding

src/ImageSharp/Formats/Pbm/PlainEncoder.cs

@@ -66,6 +66,9 @@ namespace SixLabors.ImageSharp.Formats.Pbm
             {
                 WriteBlackAndWhite(configuration, stream, image);
             }
+
+            // Write EOF indicator, as some encoders expect it.
+            stream.WriteByte(Space);
         }
 
         private static void WriteGrayscale<TPixel>(Configuration configuration, Stream stream, ImageFrame<TPixel> image)

tests/ImageSharp.Tests/Formats/Pbm/PbmEncoderTests.cs

@@ -72,7 +72,13 @@ namespace SixLabors.ImageSharp.Tests.Formats.Pbm
                 using (var memStream = new MemoryStream())
                 {
                     input.Save(memStream, options);
-                    memStream.Position = 0;
+
+                    // EOF indicator for plain is a Space.
+                    memStream.Seek(-1, SeekOrigin.End);
+                    int lastByte = memStream.ReadByte();
+                    Assert.Equal(0x20, lastByte);
+
+                    memStream.Seek(0, SeekOrigin.Begin);
                     using (var output = Image.Load<Rgba32>(memStream))
                     {
                         PbmMetadata meta = output.Metadata.GetPbmMetadata();