[BUG][C#] Dto properties can't be made non-nullable

Description

If I try to generate aspnetcore dto's the nullable = false is alwasy ignored and all the properties created are created nullable.

If I have nullable field in my api.yaml file (skipping allot for brevity)

        expireDateOfOffer:
          type: string
          format: date
          nullable: false (toggling/skipping this doesn't change anything! )

and then run the following

java -DdebugModels -jar openapi-generator-cli-4.0.0-20181226.105224-120.jar generate -i api.yaml -g aspnetcore -o c:\Temp -t templates\aspnetcore\controller -Dapis

I get

        /// <summary>
        /// Gets or Sets ExpireDateOfOffer
        /// </summary>
        [DataMember(Name="expireDateOfOffer")]
        public DateTime? ExpireDateOfOffer { get; set; }

I can't seem to get rid off the ? (nullable) after the DateTime.
I don't want it to be nullable!

openapi-generator version

4.0.0-20181226.105224-120

Command line used for generation

java -DdebugModels -jar openapi-generator-cli-4.0.0-20181226.105224-120.jar generate -i api.yaml -g aspnetcore -o c:\Temp -t templates\aspnetcore\dtos -Dapis

Steps to reproduce

Run the previous command line on a yaml file with property you want to have as non-nullable and see if you can make it so.

Related issues/PRs

Couldn't find anything except maybe this one

Reproduce the issue with the provided api.yaml and openapi-generator 4.0.0 command, using the aspnetcore DTO templates under templates/aspnetcore/dtos. Inspect the generated model and debugModels output to trace how nullable=false is handled. Done means the generated DateTime property no longer has the nullable marker when the schema property is non-nullable.