[CppRest] file properties of models not generated as shared_ptr<>

Description

After a long time using my original version of the cpprest generator, I decided to migrate finally to the official swagger-codegen version. We are using a somehow non-open-api-standard compliant feature: having files as model properties. This used to work in my original codebase but seemed to be broken since issue #3773 was fixed. Instead of generating:

std::shared_ptr<HttpContent> getFile();
    void setFile(std::shared_ptr<HttpContent> value);

...the following source is generated:

HttpContent getFile();
    void setFile(HttpContent value);

This in advance leads to errors when serializing/deserializing as the ModelBase only provides overloads for the shared_ptr.

Swagger-codegen version

2.2.3

Swagger declaration file content or url

https://gist.github.com/Danielku15/27d3604077fd325ba4dacf475a8fc66a

Command line used for generation

java -cp "swagger-codegen-cli.jar" io.swagger.codegen.SwaggerCodegen generate^
        -l cpprest^
        -i "SwaggerSpec.json"^
        -o "Generated"^
        -c "config.json"

Steps to reproduce

1. Take given swagger specification and generate a cpprest client
2. Inspect the MyFileViewModel.cpp and notice the non-shared_ptr file property.
3. Compile the sources and there will be compilation errors for MyFileViewModel::fromJson as it cannot call setFile(HttpContent) with a shared_ptr<HttpContent> parameter.
4. Also a compile error on MyFileViewModel::toMultipart is shown as there is no ModelBase::toHttpContent(utility::string_t, HttpContent) overload. It needs a shared_ptr<HttpContent> as second parameter.

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/3773
https://github.com/swagger-api/swagger-codegen/pull/3876

Suggest a fix/enhancement

I expect that removing p instanceof FileProperty from the code follow source line will fix the issue by bringing back the shared_ptr<HttpContent>. Maybe @sdrichter can point out why it was added in a first place. His issue mentions something that there were issues on APIs which return a file object.

https://github.com/swagger-api/swagger-codegen/blob/8cc8c1eac6eba2770bca17ca6387bc0303756574/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/CppRestClientCodegen.java#L308

Start in modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/CppRestClientCodegen.java at the referenced line, then generate the client with the supplied Swagger declaration and cpprest command. Inspect MyFileViewModel.cpp and verify that file properties use shared_ptr, allowing fromJson and toMultipart to compile with the ModelBase overloads.