Emit Approval events from burnFrom and transferFrom (Closes #74)

contracts/token/ERC20/ExternalERC20.sol

@@ -181,6 +181,7 @@ contract ExternalERC20 is IERC20 {
             _externalERC20Storage.allowed(from, originSender).sub(value)
         );
         _transfer(from, to, value);
+        emit Approval(from, originSender, _externalERC20Storage.allowed(from, originSender));
     }
 
     /**
@@ -283,5 +284,6 @@ contract ExternalERC20 is IERC20 {
                 _externalERC20Storage.allowed(account, burner).sub(value)
             );
         _burn(account, value);
+        emit Approval(account, burner, _externalERC20Storage.allowed(account, burner));
     }
 }

test/token/ERC20/behaviors/ERC20.public.behavior.js

@@ -186,13 +186,28 @@ function shouldBehaveLikeERC20PublicAPI (owner, recipient, anotherAccount) {
             (await this.token.allowance(owner, spender)).should.be.bignumber.equal(0);
           });
 
-          it('emits a transfer event', async function () {
-            const { logs } = await this.token.transferFrom(owner, to, amount, { from: spender });
+          describe('emits', function () {
+            let events;
 
-            expectEvent.inLogs(logs, 'Transfer', {
-              from: owner,
-              to: to,
-              value: amount
+            beforeEach(async function () {
+              const { logs } = await this.token.transferFrom(owner, to, amount, { from: spender });
+              events = logs;
+            });
+
+            it('a transfer event', async function () {
+              expectEvent.inLogs(events, 'Transfer', {
+                from: owner,
+                to: to,
+                value: amount
+              });
+            });
+
+            it('an approval event', async function () {
+              expectEvent.inLogs(events, 'Approval', {
+                owner: owner,
+                spender: spender,
+                value: 0
+              });
             });
           });
         });

test/token/ERC20/behaviors/ERC20Burnable.behavior.js

@@ -84,6 +84,14 @@ function shouldBehaveLikeERC20Burnable (owner, initialBalance, [burner]) {
             value: amount
           });
         });
+
+        it('emits an approval event', async function () {
+          expectEvent.inLogs(this.logs, 'Approval', {
+            owner: owner,
+            spender: burner,
+            value: originalAllowance - amount
+          });
+        });
       }
     });