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Description of the PR

@mhardorf @leonyu @roni-frantchi @bherila

Related to this issue. When using TypeScript Fetch and passing parameters to a "Date" field, they expect a type of "string", but then toISOString() is called on it, which causes the function to fail. Same issue on swagger-codegen.

Example json swagger:

        "parameters": [
          {
            "name": "date",
            "in": "query",
            "type": "string",
            "format": "date"
          }
        ]

in In `AbstractTypeScriptClientCodegen.java the Typings map:

typeMapping.put("date", "string");
typeMapping.put("DateTime", "Date");

template issue:

{{#isDate}}
queryParameters['{{baseName}}'] = (requestParameters.{{paramName}} as any).toISOString();
{{/isDate}}

requestParameters.{{paramName}} can never be a Date, since it has to be a string.