Refactor filter descriptor type checks in SwaggerGen

[iskandersierra]

Pull Request

Given that the public API allows to register a filter in this form without any specific validation on not assigning the Type property:

genOptions.OperationFilterDescriptors.Add(new FilterDescriptor
{
    // Type = typeof(TdpHeaderParameter),
    FilterInstance = new TdpHeaderParameter(IRequestContext.UserIdKey, "The user ID."),
});

in which case, opening the swagger UI returns a NullReferenceException (in developer error page) with no reference to user source code.

Looking into the library source code, I found out that the "proper" way to do the registration is:

genOptions.AddOperationFilterInstance(
    new TdpHeaderParameter(IRequestContext.TenantIdKey, "The tenant ID."));

but I just felt that the first option should also be available with minimal behavior changes, given that it is a possible choice in the public API.

The issue or feature being addressed

NOTE: I didn't create a specific issue

Details on the issue fix or feature implementation

I added an alternative validation on the filter type for when the FilterInstance is set but not the Type of the FilterDescriptor.

[martincostello]

Can you add a test that validates the NullReferenceException has gone away with the change?

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.44%. Comparing base (e1c5153) to head (acb7dbc).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3125   +/-   ##
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  Coverage   91.43%   91.44%           
=======================================
  Files          76       76           
  Lines        3119     3121    +2     
  Branches      519      520    +1     
=======================================
+ Hits         2852     2854    +2     
  Misses        267      267           

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