use native json marshaling to map stubbed output (#52)

* use native json marshaling to map stubbed output

When an output field had a '_' in its name, it was not mapped
correctly by 'mapstructure' to the protoc generated class because,
in this class, the fields are translated in CamelCase.
Hower, the fields have json tag with their original name.
Json marshalling is then the way to fix this issue.

Dockerfile

@@ -7,7 +7,6 @@ RUN mkdir /stubs
 RUN apk -U --no-cache add git protobuf
 
 RUN go get -u -v github.com/golang/protobuf/protoc-gen-go \
-	github.com/mitchellh/mapstructure \
 	google.golang.org/grpc \
 	google.golang.org/grpc/reflection \
 	golang.org/x/net/context \
@@ -16,7 +15,7 @@ RUN go get -u -v github.com/golang/protobuf/protoc-gen-go \
 	golang.org/x/tools/imports
 
 RUN go get -u -v github.com/gobuffalo/packr/v2/... \
-                 github.com/gobuffalo/packr/v2/packr2
+	github.com/gobuffalo/packr/v2/packr2
 
 # cloning well-known-types
 RUN git clone https://github.com/google/protobuf.git /protobuf-repo

example/simple/client/main.go

@@ -32,5 +32,5 @@ func main() {
 	if err != nil {
 		log.Fatalf("error from grpc: %v", err)
 	}
-	log.Printf("Greeting: %s", r.Message)
+	log.Printf("Greeting: %s (return code %d)", r.Message, r.ReturnCode)
 }

example/simple/simple.proto

@@ -16,4 +16,5 @@ message Request {
 // The response message containing the greetings
 message Reply {
   string message = 1;
+  int32 return_code = 2;
 }