HPCC-9135 multi-part boundary line check should include extra '-'

When uploading a file, we need to check possible mime boundary line
using the length of mime boundary ID. Browser adds two extra '-'s
in the front of mime boundary ID. If the length of the last line
inside a file content block is less than the length of mime boundary
ID plus two, the last line may still be a mime boundary line separated
by the content block.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>