Fix generate crash when have some unknown field

[wingyplus]

Found by accidentally add $schema to the json for autocompletion
reason. The Tesla.OpenApi.Spec.models/1 at line 278 will crash because
of for iterate defs over map which's cause schema to be the value of
field $schema. Fixes by filter only schema that is map.

[wingyplus]

This is error that I found:

Compiling 1 file (.ex)

== Compilation error in file lib/diablo_clone.ex ==
** (FunctionClauseError) no function clause matching in Access.get/3    
    
    The following arguments were given to Access.get/3:
    
        # 1
        "https://raw.githubusercontent.com/OAI/OpenAPI-Specification/main/schemas/v3.1/schema.json"
    
        # 2
        "title"

        # 3
        nil

    Attempted function clauses (showing 5 out of 5):

        def get(-%module{} = container-, +key+, +default+)
        def get(+map+, +key+, +default+) when -is_map(map)-
        def get(+list+, +key+, +default+) when -is_list(list)- and -is_atom(key)-
        def get(+list+, +key+, +_default+) when -is_list(list)-
        def get(-nil-, +_key+, +default+)

    (elixir 1.13.4) lib/access.ex:283: Access.get/3
    (tesla_openapi 0.1.0) lib/tesla/openapi/spec.ex:281: anonymous fn/2 in Tesla.OpenApi.Spec.models/1
    (stdlib 3.17.2) maps.erl:410: :maps.fold_1/3
    (tesla_openapi 0.1.0) lib/tesla/openapi/spec.ex:278: Tesla.OpenApi.Spec.models/1
    (tesla_openapi 0.1.0) lib/tesla/openapi/spec.ex:202: Tesla.OpenApi.Spec.new/1
    (tesla_openapi 0.1.0) lib/tesla/openapi.ex:120: Tesla.OpenApi.generate/2
    (tesla_openapi 0.1.0) expanding macro: Tesla.OpenApi.__using__/1
    lib/diablo_clone.ex:2: DiabloClone (module)

[wingyplus]

This is openapi file that I found an error: diablo_clone.zip