Fix doctest errors

#![feature(convert)]

extern crate pkg_config;

extern crate gcc;

/// let data = b"what do ya want for nothing?";

/// let spec = b"\x75\x0c\x78\x3e\x6a\xb0\xb5\x03\xea\xa8\x6e\x31\x0a\x5d\xb7\x38";

/// let res = hmac(Type::MD5, key, data);

/// assert_eq!(spec, res);

/// assert_eq!(res, spec);

/// ```

///

/// Use the `Write` trait to supply the input in chunks.

/// h.write_all(data[0]);

/// h.write_all(data[1]);

/// let res = h.finish();

/// assert_eq!(spec, res);

/// assert_eq!(res, spec);

/// ```

pub struct HMAC {

    ctx: ffi::HMAC_CTX,