Add support for ToRedisArgs for &std::string::String

[redis-rs]

Add support for ToRedisArgs for &std::string::String

[badboy]

connection.pfadd(
     &my_key[..],
     &my_value[..],
)

should just work, as we already have ToRedisArgs for &str. But because it is generic &String does not coerce to &str here.
See https://github.com/mitsuhiko/redis-rs/blob/dfa47519578e19389873dbbe57f2cbf8dfc9a79c/src/types.rs#L497

The following would be necessary to support &String without any more user help:

impl<'a> ToRedisArgs for &'a String {
    fn to_redis_args(&self) -> Vec<Vec<u8>> {
        vec![self.as_bytes().to_vec()]
    }
}

@mitsuhiko: What do you think?

[mitsuhiko]

Sounds like a reasonable thing to add.