[C#] Fixed invalid generation of C# children models in allOf clauses

Created by: rubms

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Fixed the #1126 (closed) issue, that causes the auto-generated C# children models not to compile because they attempt to invoke a BaseValidate method in the parent model that is not implemented. In order to fix the issue:

• DefaultCodegen.java now sets the hasChildren of CodegenModel to true in models that have children.
• The modelGeneric.mustache now uses hasChildren, instead of discriminator, in order to decide whether to implement a BaseValidate method (for children to run the parent model validations). In addition, now the BaseValidate method of the parent is explicitly invoked (in order to prevent infinite recursion in models that are both, children and parents.

c.c. Core (as DefaultCodegen.java has been modified). @wing328 (2015/07) @jimschubert (2016/05) @cbornet (2016/05) @jaz-ah (2016/05) @ackintosh (2018/02) @JFCote (2018/03) @jmini (2018/04)

c.c. C# @mandrean (2017/08) @jimschubert (2017/09)