More consistency between TLS implementations

While GnuTLS and SecureTransport haven't shown in practice to need this,
this makes it more robust for future changes in those. In theory, both
could return less data on a read than they have buffered, meaning the
delimiter is not found but in the buffered data, which would then make
them have the same issue OpenSSL had with hanging connections (though
there the problem was that the BIO was not processed and never would
without the same change as in this commit).