[BUG] cpp-restsdk generator produces broken code

[Findus79]

Description

When generating the interface for the sample json using cpp-restsdk generator, the resulting cpp is not compilable. One error is the generated helper function

template <typename T>
utility::string_t toString(const T value)
{
    std::ostringstream out;
    out << std::setprecision(std::numeric_limits<T>::digits10) << std::fixed << value;
    return out.str();
}

where std::ostringstream out; should be utility::ostringstream_t out;

The main problems are with the generated HttpContent class. A lot of non-existing method calls are generated, like fromJson, fromMultipart, get, toMultipart. Also non-existing constructors are called on HttpContent.

openapi-generator version

tried with 3.3.4 and 4.0.0 (latest master)

OpenAPI declaration file content or url

gist link to the json file:
https://gist.github.com/Findus79/83044968492b225968c2c4bb749d2e21

Command line used for generation

java -jar openapi-generator-cli-3.4.4.jar generate -g cpp-restsdk -i input\openapi-cpprestsdk.json -o output

and

java -jar openapi-generator-cli-4.0.0.jar generate -g cpp-restsdk -i input\openapi-cpprestsdk.json -o output

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