Merge branch 'fix-monthly-periods' into 'master'

Fixed monthly periods.

See merge request julia-packages/periods.jl!3

src/SimplePeriods.jl

@@ -26,7 +26,7 @@ function Period(arg1::Integer, arg2::Integer, frequency::Frequency)
         @assert(arg2 in 1:4)
         ordinal = 4*(arg1 - 1970) + arg2 - 1
     elseif frequency == Month
-        @assert(subperiod in 1:12)
+        @assert(arg2 in 1:12)
         ordinal = 12*(arg1 - 1970) + arg2 - 1
     elseif frequency == Week
         ordinal = week1970(arg1, arg2)