Ignore two leading dashes from extracted boundary string

Some cameras returns "bad" boundary in HTTP header (with leading two dashes). Ignore them when doing boundary extraction.

eg.: http://75.129.81.154/nphMotionJpeg?Resolution=640x480&Quality=Standard

HTTP/1.0 200 OK
Content-type: multipart/x-mixed-replace; boundary=--myboundary

--myboundary
Content-length: 30427
Content-type: image/jpeg

[...jpeg frame content...]