Changed checksum calculation in Message and MessageBuilder to include Body length tag

I am certainly not a FIX protocol export, so I may be wrong here, but I believe the body length tag needs to be included in the checksum calculation.  The server I am integrating with rejected my messages until I made this change, and references to the spec indicate that only the checksum tag itself should be excluded.  For example:

http://www.onixs.biz/fix-dictionary/4.2/app_b.html