Fixed casting problems in ExecuteUtils breaking EnumerableHelpers

[HenrikHoyer]

• Average, Min, Max and Sum functions in EnumerableHelpers was broken due to casting bug in ExecuteUtils
• Improved error messages
• Added Tests using these methods

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 47.27%. Comparing base (2afe410) to head (e50ff49).

Files	Patch %	Lines
src/Handlebars.Net.Helpers/Utils/ExecuteUtils.cs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
+ Coverage   46.29%   47.27%   +0.97%     
==========================================
  Files          44       44              
  Lines        2912     2913       +1     
  Branches      410      411       +1     
==========================================
+ Hits         1348     1377      +29     
+ Misses       1440     1412      -28     
  Partials      124      124              

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[StefH]

@HenrikHoyer

1. You can mark the comment comments which are fixed as "resolve conversation", this makes it easier to see which one are resolved or still open.

2. There is a unit-test error:
   Expected result to be 450.6, but found 450.60000038146973.
   Use something like this code:
   result.Should().BeApproximately(450.6, 0.0001);