[BUG][typescript-angular] Type mapping from DateTime to string causes build error

[mmajrk]

I encountered the same issue on v 4.1.3 as described here:
#2745

When using a type mapping from DateTime to string, generated code contains lines such as queryParameters = queryParameters.set('dateFrom', <any>dateFrom.toISOString());, which causes build failure.

This was fixed for typescript-axios generator, but the issue is still present in typescript-angular. I'd like to open a PR with a fix.

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[mmajrk]

I'm not familiar with the codebase, would it be ok to override DefaultCodegen.fromProperty in AbstractTypeScriptClientCodegen.java? Or is it better to perform a runtime type check in the template as in #3423?

@Override
public CodegenProperty fromProperty(String name, Schema p) {
    CodegenProperty property = super.fromProperty(name, p);
    if (property.isDateTime && (property.dataType == "string" || property.dataType == "String")) {
        property.isString = true;
        property.isDateTime = false;
    }
    return property;
}

[macjohnny]

I think a runtime check is better