PricerRule convertPayCurrencyToToken method missing factor fix

[abhayks1]

The PR adds missing decimals factor in Function convertPayCurrencyToTokens.

Added below unit tests:

• requiredPriceOracleDecimals = 18 and eip20TokenDecimals = 18
• requiredPriceOracleDecimals = 5 and eip20TokenDecimals = 18
• requiredPriceOracleDecimals = 18 and eip20TokenDecimals = 5

Theoretically, the Number of BT needs to be transferred for payment shouldn’t depend on requiredPriceOracleDecimals value. If 20 dollar payment needs to be done, it would require 1000 BT for $0.02 per BT value. Number of BT needs to be transferred shouldn't depend on price oracle decimal.

requiredPriceOracleDecimals simply decides minimum value in currency(say USD) that can be transferred.

Below two test cases have different price oracle decimals. For test case 1st, requiredPriceOracleDecimals is 18 and the number of BT which will be transferred after the calculation is 1000BT which is expected.

Total value in USD = 1000 * 0.02 = 20$ (where value of each BT is 0.02$)

However for test case two, requiredPriceOracleDecimals is 5 and the number of calculated BT is 10^8 wei BT.

Total value in USD = (10^8 * 0.02) / (10^18) = 2*10 ^ (-12) $
Case 1

Input :

requiredPriceOracleDecimals = 18
payCurrencyAmount = $20 = 20*10^18
baseCurrencyPriceInPayCurrency = $0.02 = 0.02 * 10^18
conversionRateFromBaseCurrencyToToken = 100000 (For 1 OST = 1 BT)
conversionRateDecimalsFromBaseCurrencyToToken = 5
eip20TokenDecimals = 18

Expected Transferred Amount:

1000 BT = 1000 * 10^18

Transferred Amount with Pay method:

1000 BT = 1000 * 10^18

Unit Test Status:

Pass

Case 2

Input :

requiredPriceOracleDecimals = 5
payCurrencyAmount = $20 = 20*10^5
baseCurrencyPriceInPayCurrency = $0.02 = 0.02 * 10^5
conversionRateFromBaseCurrencyToToken = 100000 (For 1 OST = 1 BT)
conversionRateDecimalsFromBaseCurrencyToToken = 5
eip20TokenDecimals = 18

Expected Transferred Amount:

1000 BT = 1000 * 10^18 BTWei

Transferred Amount with Pay method:

10^8 BTWei

Unit Test Status:

Fail
Pass

Travis CI Result:
https://travis-ci.org/OpenSTFoundation/openst-contracts/builds/499911526?utm_source=github_status&utm_medium=notification

Calculation with BT decimal = 20

1 OST = 1 BT
BT CR = 10^5
BT CRD = 5
BT Decimal = 20
USD Price Oracle decimal = 18
OST Base Currency decimal = 18

1 OST = 0.02 USD
Transfer 20 USD

Expected Output: 
	1000 BT = 1000*10^18 BTWei

Current PricerRule Implementation Output:

(20*10^18 * 10^5) / (0.02 * 10^18 * 10^5) = 1000 (BT)

Abhay Calculation Output:

(20*10^18 * 10^5) * 10^20 / (0.02 * 10^18 * 10^5) = 1000 * 10^20 BTWei (1000 BT if BT decimal is considered)
As BT Decimal = 10^20, 1000 * 10^20 units = 1000 * 10^20/10^20 = 1000 BT

Gist Discussion:
https://gist.github.com/abhayks1/ca49a7db6c0e89ddf4dd9ba568abfac1

cc: @benjaminbollen @pgev @schemar @sarvesh-ost