Using discardSomeReadBytes instead of discardReadBytes for CommandHan…

…dler's buffer

Fixes redis#725.

src/main/java/io/lettuce/core/protocol/CommandHandler.java

@@ -562,7 +562,7 @@ protected void decode(ChannelHandlerContext ctx, ByteBuf buffer) throws Interrup
             }
 
             if (buffer.refCnt() != 0) {
-                buffer.discardReadBytes();
+                buffer.discardSomeReadBytes();
             }
 
             afterComplete(ctx, command);