fix the investor count (#366)

Author: satyamakgec

contracts/modules/STO/PreSaleSTO.sol

@@ -77,8 +77,10 @@ contract PreSaleSTO is ISTO {
         require(now <= endTime, "Current time should less than the endTime");
         require(_amount > 0, "No. of tokens provided should be greater the zero");
         ISecurityToken(securityToken).mint(_investor, _amount);
+        if (investors[_investor] == uint256(0)) {
+            investorCount = investorCount.add(1);
+        }
         investors[_investor] = investors[_investor].add(_amount);
-        investorCount = investorCount.add(1);
         fundsRaised[uint8(FundRaiseType.ETH)] = fundsRaised[uint8(FundRaiseType.ETH)].add(_etherContributed);
         fundsRaised[uint8(FundRaiseType.POLY)] = fundsRaised[uint8(FundRaiseType.POLY)].add(_polyContributed);
         totalTokensSold = totalTokensSold.add(_amount);

test/m_presale_sto.js

@@ -352,7 +352,13 @@ contract("PreSaleSTO", accounts => {
             );
         });
 
-        it("Should failed at the time of buying the tokens -- Because STO has started", async () => {
+        it("Should buy some more tokens to previous investor", async() => {
+            await I_PreSaleSTO.allocateTokens(account_investor1, web3.utils.toWei("1000", "ether"), web3.utils.toWei("1", "ether"), 0, { from: account_issuer });
+            // No change in the investor count
+            assert.equal((await I_PreSaleSTO.getNumberInvestors.call()).toNumber(), 3);
+        })
+
+        it("Should failed at the time of buying the tokens -- Because STO has ended", async () => {
             await increaseTime(duration.days(100)); // increased beyond the end time of the STO
 
             await catchRevert(