handle arrays of refs in parameter schemas

[fenollp]

A parameter such as

      entries(:body, Schema.array(:BlueprintEntry), "a blueprint's entries", required: false)

generates this Swagger2 parameter:

           {
             "schema": {
               "type": "array",
               "items": {
                 "$ref": "#/definitions/BlueprintEntry"
               }
             },
             "required": false,
             "name": "entries",
             "in": "body",
             "description": "a blueprint's entries"
           }

which is not handled by the validation part of this lib:

    ** (EXIT) an exception was raised:
        ** (FunctionClauseError) no function clause matching in String.split/3
            (elixir) lib/string.ex:407: String.split(nil, "/", [])
            (phoenix_swagger) lib/phoenix_swagger/validator.ex:111: anonymous fn/3 in PhoenixSwagger.Validator.collect_schema_attrs/1
            (elixir) lib/enum.ex:1925: Enum."-reduce/3-lists^foldl/2-0-"/3
            (phoenix_swagger) lib/phoenix_swagger/validator.ex:109: anonymous fn/4 in PhoenixSwagger.Validator.collect_schema_attrs/1
            (elixir) lib/enum.ex:1314: Enum."-map/2-lists^map/1-0-"/2
            (elixir) lib/enum.ex:1314: Enum."-map/2-lists^map/1-0-"/2
            (elixir) lib/enum.ex:1318: anonymous fn/3 in Enum.map/2
            (stdlib) maps.erl:257: :maps.fold_1/3

This patch is my attempt at lowering the validator's assumptions on schemas/types in parameters, however it fails a couple tests... Ideas?

[mbuhot]

I can narrow it down to this line causing the test failures