Fix write_header method to use dup

Author: kojix2

lib/hts/bcf.rb

@@ -98,7 +98,7 @@ def close
     def write_header(header)
       check_closed
 
-      @header = header.clone
+      @header = header.dup
       LibHTS.bcf_hdr_write(@hts_file, header)
     end