Suppress a warning.

Suppress a warning.

The compiler warns about the line
  header_ += gzip::magic::id2;                   // ID2.
deep in BOOST when writing the header for a gzip-compressed block of data. This
is because (i) gzip::magic::id2 is declared as an int, and (ii) has a value
greater than what a *signed char* can store, i.e., greater than 127. Furthermore,
std::string::operator+ takes a 'char', which may or may not be unsigned but
apparently on my system is signed. So we get a warning about overflow.

The only reasonable way to deal with this is to do the casting explicitly.