Android volley library enhancement and tests

[Shyri]

Hi! I have added some enhancements to android-volley-library generator. I want to add tests requested in #1839 but I have some questions so I am gonna make a to do list:

• Add Synchronous Requests support
• Improve RequestQueue configuration capabilities
• Write units test for PetApi.

I will explain one by one:

Add Synchronous Requests support

The most common usage of Volley is making asynchronous requests with callbacks as there were already in the volley generator, something like:

api.getPetById(pet.getId(), 
    new Response.Listener<Pet>() {
            @Override
            public void onResponse(Pet response) {
                fetched[0] = response;
                waiter.resume();
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

            }
        });

But there are cases, like in services for instance, where you want to call that request synchronously. I've added this functionality so one can call both the way above or like android-default was:

Pet fetched = api.getPetById(pet.getId());

I've set a 30 seconds timeout, I don't know if this is ok with you. Should I make it configurable?

Improve RequestQueue configuration capabilities

With current version is necessary to provide a Context to ApiInvoker initializer so it can build a RequestQueue based on that Context. I've added to the initializer some parameters to configure the Volley request queue like:
ApiInvoker(Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery). These are passed to RequestQueue constructor. Default would do something equivalent to:

``` java`
HttpStack stack = new HurlStack();
network = new BasicNetwork(stack);
cache = new NoCache();
mRequestQueue = new RequestQueue(cache, network)

that will use a default network thread pool size of 4, and a 

``` java
new ExecutorDelivery(new Handler(Looper.getMainLooper()))

as ResponseDelivery

Unit Tests

I've been testing the petstore.json.orig file. There are some issues: As OAuth2 support is not yet implemented I cannot get it working straight out of the generated code, as it expects to receive some value for auth keys. Is this the correct the correct behaviour? Should the client throw an exception when no authentication was provided but it is specified, like in deletePet

...
"security": [
          {
            "petstore_auth": [
              "write:pets",
              "read:pets"
            ]
          }
        ]
...

or should it let the request be made and let the server return failure?

Anyway, I've written the PetApiTest both for sync and async apis from a modified json removing oauth and adding apikey authentication in all methods. Once I have this clear I'll make the necessary changes and write the rest of the tests

[wing328]

About timeout, yes please make it configurable. C#, Ruby, PHP, Java, etc allows developers to set the timeout.

[wing328]

RE: should it let the request be made and let the server return failure?

General speaking, if the authentication setting (e.g. username, password, token, API key,etc) is not passed to the method via Configuration or other ways (e.g. set default header), then the API client (in Ruby, PHP, Java, etc) will still make the request and the server will send back an error (e.g. 401), which will be handled by the API client and throw an Api Exception accordingly.

[wing328]

To add OAuth support to the Android client, you may want to have a look at at the Java Retrofit client: https://github.com/swagger-api/swagger-codegen/tree/master/samples/client/petstore/java/retrofit/src/main/java/io/swagger/client/auth

[wing328]

For petstore.json, other API clients are using https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/test/resources/2_0/petstore.json for unit testing.

I'm not familiar with petstore.json.org so I'm not sure if it's correct.

[Shyri]

I've added configurable timeout, some fix for apikey check in order to let the request to be sent when no apikey was set.
Also I've added the PetApiTest unit tests and an OAuth stub so the tests can pass. If you agree I'm going to add OAuth support to the checklist so we can have a full featured generator out of this PR

[wing328]

Thanks @Shyri I suggest we merge these changes into master now so as to gather feedback from the developers while you're working on the remaining items.

[Shyri]

@wing328 That's fine by me :)