Fixes a bug that was not returning the hash

The way this library works is that it is demand-driven, not
supply-driven; i.e. it will only fill as many bytes as you provide in a
given slice. The prior implementation was a vector of length 0 so the
backing slice requested no bytes.

beacon_chain/utils/hashing/src/lib.rs

@@ -5,7 +5,26 @@ use tiny_keccak::Keccak;
 pub fn canonical_hash(input: &[u8]) -> Vec<u8> {
     let mut keccak = Keccak::new_keccak256();
     keccak.update(input);
-    let mut result = Vec::with_capacity(32);
+    let mut result = vec![0; 32];
     keccak.finalize(result.as_mut_slice());
     result
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::convert::From;
+
+    #[test]
+    fn test_hashing() {
+        let input: Vec<u8> = From::from("hello");
+
+        let output = canonical_hash(input.as_ref());
+        let expected = &[
+            0x1c, 0x8a, 0xff, 0x95, 0x06, 0x85, 0xc2, 0xed, 0x4b, 0xc3, 0x17, 0x4f, 0x34, 0x72,
+            0x28, 0x7b, 0x56, 0xd9, 0x51, 0x7b, 0x9c, 0x94, 0x81, 0x27, 0x31, 0x9a, 0x09, 0xa7,
+            0xa3, 0x6d, 0xea, 0xc8,
+        ];
+        assert_eq!(expected, output.as_slice());
+    }
+}