Fix possible UndefinedFunctionError in ExAws.InstanceMeta.request/2

[ishikawa]

ExAws.Request.HttpClient behaviour specifies one callback function.

  @callback request(
              method :: http_method,
              url :: binary,
              req_body :: binary,
              headers :: [{binary, binary}, ...],
              http_opts :: term
            ) ::
              {:ok, %{status_code: pos_integer, body: binary}}
              | {:error, %{reason: any}}

But in ExAws.InstanceMeta.request/2, it expects HttpClient.reqeust/2. So an UndefinedFunctionError will be occurred if a custom HttpClient module doesn't implement request/2.

** (UndefinedFunctionError) function MyHTTPClient.request/2 is undefined or private
(my_project) MyHTTPClient.request(:get, "http://169.254.169.254/latest/meta-data/iam/security-credentials/")
(ex_aws) lib/ex_aws/instance_meta.ex:16: ExAws.InstanceMeta.request/2
(ex_aws) lib/ex_aws/instance_meta.ex:58: ExAws.InstanceMeta.instance_role_credentials/1
(ex_aws) lib/ex_aws/instance_meta.ex:64: ExAws.InstanceMeta.security_credentials/1
(ex_aws) lib/ex_aws/config/auth_cache.ex:58: ExAws.Config.AuthCache.refresh_config/2
(ex_aws) lib/ex_aws/config/auth_cache.ex:33: ExAws.Config.AuthCache.handle_call/3
(stdlib) gen_server.erl:661: :gen_server.try_handle_call/4
(stdlib) gen_server.erl:690: :gen_server.handle_msg/6

This PR fixes the issue and add a simple test.

[benwilson512]

Thanks!