Updated the OCaml/bitwriter.ml test for OCaml 4.06+

Updated the OCaml/bitwriter.ml test for OCaml 4.06+

Since OCaml 4.02 (released in 2014), strings and bytes are different
types, but up until OCaml 4.06, the compiler defaulted to a
compatibility mode "unsafe-string". OCaml 4.06 flips the default to
"safe-string", breaking the test.

This change should be compatible with OCaml 4.02+, but is only truly
necessary for OCaml 4.06+.

For more information, see:

https://caml.inria.fr/pub/docs/manual-ocaml/libref/String.html
https://ocaml.org/releases/4.02.html