Fix schema initialization exceptions

[Shane32]

Fixes #3444

See reports of this issue:

• Custom JsonConverter for DateTime types not being called when serializing the response #3359 (comment)
• Required service for type GraphQL.Types.LongGraphType not found occurred #3374
• After updating to 7.0.0, problems with executing operations that include union types conventions#234

Also cc @STRATZ-Ken who also experienced this issue

[codecov-commenter]

Codecov Report

Merging #3445 (79eb972) into master (997dfaa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3445   +/-   ##
=======================================
  Coverage   83.71%   83.71%           
=======================================
  Files         377      377           
  Lines       16392    16399    +7     
  Branches     2640     2641    +1     
=======================================
+ Hits        13722    13729    +7     
  Misses       2049     2049           
  Partials      621      621           

Impacted Files	Coverage Δ
src/GraphQL/Types/Schema.cs	72.50% <100.00%> (+0.78%)	⬆️

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[sungam3r]

Nice fix.