Be a little smarter about our detection of the placed sign

When placing a sign against a tile entity (such as a turtle or chest),
we would consider that block the "placed" one instead, meaning the text
was never set. This solution isn't entirely ideal either, but short of
capturing block snapshots I'm not sure of a better solution.

Fixes #552

src/main/java/dan200/computercraft/shared/turtle/core/TurtlePlaceCommand.java

@@ -365,7 +365,7 @@ private static ItemStack deployOnBlock( @Nonnull ItemStack stack, ITurtleAccess
 
         // Do the deploying (put everything in the players inventory)
         boolean placed = false;
-
+        TileEntity existingTile = turtle.getWorld().getTileEntity( position );
 
         // See PlayerInteractionManager.processRightClickBlock
         PlayerInteractEvent.RightClickBlock event = ForgeHooks.onRightClickBlock( turtlePlayer, EnumHand.MAIN_HAND, position, side, new Vec3d( hitX, hitY, hitZ ) );
@@ -409,12 +409,11 @@ else if( actionResult == null )
             {
                 World world = turtle.getWorld();
                 TileEntity tile = world.getTileEntity( position );
-                if( tile == null )
+                if( tile == null || tile == existingTile )
                 {
-                    BlockPos newPosition = WorldUtil.moveCoords( position, side );
-                    tile = world.getTileEntity( newPosition );
+                    tile = world.getTileEntity( WorldUtil.moveCoords( position, side ) );
                 }
-                if( tile != null && tile instanceof TileEntitySign )
+                if( tile instanceof TileEntitySign )
                 {
                     TileEntitySign signTile = (TileEntitySign) tile;
                     String s = (String)extraArguments[0];