[core] Support unusual piece sizes (again?)

I unintentionally removed support for piece sizes which
were not multiples of 16kB. It  came as a surprise to
learn these were not officially forbidden by the spec,
the spec just calls for 'usually a multiple of 2'.

Strange.

src/MonoTorrent.Tests/Client/StandardPickerTests.cs

@@ -497,6 +497,18 @@ public void CreateNormalPiece ()
             Assert.AreEqual (Piece.BlockSize * 1, piece.Blocks[2].RequestLength);
         }
 
+        [Test]
+        public void CreatePiece_NotMultipleOf16KB ()
+        {
+            var totalSize = 2318336;
+            var piece = new Piece (0, totalSize);
+            Assert.AreEqual (142, piece.BlockCount);
+
+            for (int i = 0; i < piece.BlockCount - 1; i++)
+                Assert.AreEqual (Piece.BlockSize, piece[i].RequestLength);
+            Assert.AreEqual (totalSize - (Piece.BlockSize * 141), piece[141].RequestLength);
+        }
+
         [Test]
         public void PickBundle ()
         {

src/MonoTorrent.Tests/Client/TorrentDataExtensionsTests.cs

@@ -52,6 +52,8 @@ public void BlocksPerPiece ()
             Assert.AreEqual (1, new Data { Size = Piece.BlockSize * 5 - 1, PieceLength = Piece.BlockSize * 2 }.BlocksPerPiece (2));
 
             Assert.AreEqual (2, new Data { Size = (long) (int.MaxValue) * 4, PieceLength = Piece.BlockSize * 2 }.BlocksPerPiece (0));
+
+            Assert.AreEqual (142, new Data { Size = 16 * 1024 * 1024, PieceLength = 2318336 }.BlocksPerPiece (0));
         }
 
         [Test]

src/MonoTorrent/MonoTorrent/ITorrentData.cs

@@ -54,7 +54,7 @@ static class ITorrentDataExtensions
         public static int BlocksPerPiece (this ITorrentData self, int pieceIndex)
         {
             if (pieceIndex < self.PieceCount () - 1)
-                return self.PieceLength / Piece.BlockSize;
+                return (Piece.BlockSize - 1 + self.PieceLength) / Piece.BlockSize;
 
             var remainder = self.Size - self.PieceIndexToByteOffset (pieceIndex);
             return (int) ((remainder + Piece.BlockSize - 1) / Piece.BlockSize);