Fix proof-of-possession issues.

These were introduced in an earlier commit

eth2/types/src/deposit_input.rs

@@ -1,5 +1,5 @@
-use super::Hash256;
 use crate::test_utils::TestRandom;
+use crate::*;
 use bls::{Keypair, PublicKey, Signature};
 use rand::RngCore;
 use serde_derive::{Deserialize, Serialize};
@@ -37,14 +37,30 @@ impl DepositInput {
         withdrawal_credentials: &Hash256,
         domain: u64,
     ) -> Signature {
-        let signable_deposite_input = DepositInput {
+        let signable_deposit_input = DepositInput {
             pubkey: keypair.pk.clone(),
             withdrawal_credentials: withdrawal_credentials.clone(),
             proof_of_possession: Signature::empty_signature(),
         };
-        let msg = signable_deposite_input.signed_root();
+        let msg = signable_deposit_input.signed_root();
+
         Signature::new(msg.as_slice(), domain, &keypair.sk)
     }
+
+    /// Verify that proof-of-possession is valid.
+    ///
+    /// Spec v0.4.0
+    pub fn validate_proof_of_possession(
+        &self,
+        epoch: Epoch,
+        fork: &Fork,
+        spec: &ChainSpec,
+    ) -> bool {
+        let msg = self.signed_root();
+        let domain = spec.get_domain(epoch, Domain::Deposit, fork);
+
+        self.proof_of_possession.verify(&msg, domain, &self.pubkey)
+    }
 }
 
 #[cfg(test)]

eth2/types/src/test_utils/testing_beacon_block_builder.rs

@@ -153,12 +153,18 @@ impl TestingBeaconBlockBuilder {
     }
 
     /// Insert a `Valid` deposit into the state.
-    pub fn insert_deposit(&mut self, amount: u64, index: u64, domain: u64, spec: &ChainSpec) {
+    pub fn insert_deposit(
+        &mut self,
+        amount: u64,
+        index: u64,
+        state: &BeaconState,
+        spec: &ChainSpec,
+    ) {
         let keypair = Keypair::random();
 
         let mut builder = TestingDepositBuilder::new(amount);
         builder.set_index(index);
-        builder.sign(&keypair, domain, spec);
+        builder.sign(&keypair, state, spec);
 
         self.block.body.deposits.push(builder.build())
     }

eth2/types/src/test_utils/testing_deposit_builder.rs

@@ -30,10 +30,14 @@ impl TestingDepositBuilder {
         self.deposit.index = index;
     }
 
-    pub fn sign(&mut self, keypair: &Keypair, domain: u64, spec: &ChainSpec) {
+    pub fn sign(&mut self, keypair: &Keypair, state: &BeaconState, spec: &ChainSpec) {
         let withdrawal_credentials = Hash256::from_slice(
             &get_withdrawal_credentials(&keypair.pk, spec.bls_withdrawal_prefix_byte)[..],
         );
+
+        let epoch = state.current_epoch(spec);
+        let domain = spec.get_domain(epoch, Domain::Deposit, &state.fork);
+
         self.deposit.deposit_data.deposit_input.pubkey = keypair.pk.clone();
         self.deposit
             .deposit_data