[C++][pistache] wrong code generated with the current pistache version due a namespace mismatch

Description

When I try to compile the generated code it give me the following error:

swagger-codegen/samples/server/petstore/pistache-server/api/PetApi.h:82:57: error: ‘Optional’ does not name a type
     virtual void delete_pet(const int64_t &petId, const Optional<Net::Http::Header::Raw> &apiKey, Pistache::Http::ResponseWriter &response) = 0;

Swagger-codegen version

2.4

Swagger declaration file content or url

Command line used for generation

./bin/pistache-server-petstore.sh

Steps to reproduce

1. Install pistache (https://github.com/oktal/pistache)
2. copy json.hpp (from https://github.com/nlohmann/json) to model directory (samples/server/petstore/pistache-server/model)
3. mkdir build && cd build
4. cmake ..
5. make

Related issues/PRs

Suggest a fix/enhancement

Optional namespace has changed. Now it is Pistache::Optional.
Also Http::Header::Raw has change from Net::Http::Header::Raw to Pistache::Http::Header::Raw

With this patch the code generated compiles.
https://github.com/migarma/swagger-codegen/commit/1bb0f0d2f150b012ae249b5d5378034425b061d9#diff-2c140be9bcc3e3a733ac044a8a10992b

Start with samples/server/petstore/pistache-server/api/PetApi.h and run ./bin/pistache-server-petstore.sh, then follow the cmake and make reproduction steps. Compare the generated Pistache namespace references with the current Pistache version and the linked patch. Done means the generated petstore server compiles successfully.