Fix to bug #585, http://bugs.mariadb.com/show_bug.cgi?id=585
buffer.c:gwbuf_make_contiguous: when new buffer is created, copy original buffer's type to new buffer
modutil.c:modutil_replace_SQL: when new bugger is added to GWBUF, copy original buffer's type to new buffer
In all filters, merge all buffers of GWBUF to one before calling modutil:extract_SQL