Addresses GCC 8 warning:
common_auth.cpp: In function ‘std::__cxx11::string s3fs_sha256sum(int, off_t, ssize_t)’:
common_auth.cpp:84:12: warning: ‘char* strncat(char*, const char*, size_t)’ output may be truncated copying 2 bytes from a string of length 2 [-Wstringop-truncation]
strncat(sha256, hexbuf, 2);
This allows retries of multi-part uploads instead of discovering a
fatal error during complete multipart upload. Also enable Content-MD5
for integration tests and refactor hexadecimal code.