Made PreBodyTagFilter more robust

So that it will now detect and handle accordingly the following
situations:
- only 1 </body> tag (will be prefixed with snippet if needed)
- multiple </body> tags (only last want will be prefixed with snippet it
needed)
- no </body> tags
- detect non standard casing of </body> tags (</BODY>,</BoDy>...)
Merge pull request #456 from Glimpse/FixBodyTagReplaceBehaviorInPreBodyTagFilter

Made PreBodyTagFilter more robust, Fixes issues #341 (partially) and #442 (completely)