Allow ACCRINT to accept a firstinterest date equal to settlement date. Fix #22

Author: jcoliz

src/ExcelFinancialFunctions/bonds.fs

@@ -148,7 +148,7 @@ module internal Bonds =
         accrIntM issue settlement rate par basis
     let calcAccrInt issue firstInterest settlement rate par (frequency:Frequency) basis (calcMethod:AccrIntCalcMethod) =
         (settlement > issue)            |> elseThrow "settlement must be after issue"
-        (firstInterest > settlement)    |> elseThrow "firstInterest must be after settlement"
+        (firstInterest >= settlement)   |> elseThrow "firstInterest must be after settlement"
         (rate > 0.)                     |> elseThrow "rate must be more than 0"
         (par > 0.)                      |> elseThrow "par must be more than 0"
         accrInt issue firstInterest settlement rate par frequency basis calcMethod

tests/ExcelFinancialFunctions.Tests/testdata/accrint.test

@@ -1920,3 +1920,4 @@
 4/2/1999 12:00:00 AM,1/2/2002 12:00:00 AM,7/2/2000 12:00:00 AM,0.1,12030.34,Quarterly,UsPsa30_360,1503.7925
 3/4/1984 12:00:00 AM,3/4/1994 12:00:00 AM,4/5/1991 12:00:00 AM,0.07,120,Quarterly,UsPsa30_360,59.52333333333
 3/4/1984 12:00:00 AM,3/4/1994 12:00:00 AM,4/5/1991 12:00:00 AM,0.07,120,Quarterly,UsPsa30_360,59.52333333333
+8/15/2018 12:00:00 AM,2/15/2019 12:00:00 AM,2/15/2019 12:00:00 AM,0.02125,100,SemiAnnual,ActualActual,1.0625