Fix error when indexing StepRange with begin and end

[bbejanov]

julia> (2020Q1:2:2022Q4)[end]
ERROR: promotion of types Duration{Quarterly{3}} and Duration{Quarterly{3}} failed to change any arguments
Stacktrace:
 [1] error(::String, ::String, ::String)
   @ Base ./error.jl:44
 [2] sametype_error(input::Tuple{Duration{Quarterly{3}}, Duration{Quarterly{3}}})
   @ Base ./promotion.jl:417
 [3] not_sametype(x::Tuple{Duration{…}, Duration{…}}, y::Tuple{Duration{…}, Duration{…}})
   @ Base ./promotion.jl:411
 [4] *(a::Duration{Quarterly{3}}, b::Duration{Quarterly{3}})
   @ Base ./int.jl:1067
 [5] getindex(v::StepRange{MIT{Quarterly{3}}, Duration{Quarterly{3}}}, i::Duration{Quarterly{3}})
   @ Base ./range.jl:944
 [6] top-level scope
   @ REPL[3]:1
Some type information was truncated. Use `show(err)` to see complete types.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.50%. Comparing base (726ec66) to head (eb420e0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   93.60%   93.50%   -0.11%     
==========================================
  Files          24       24              
  Lines        4849     4851       +2     
==========================================
- Hits         4539     4536       -3     
- Misses        310      315       +5     

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