Don't allow form parameters with non-form consumes

Closes #270.

tests/test_form_parameters.py

@@ -0,0 +1,46 @@
+import pytest
+from django.conf.urls import url
+from django.utils.decorators import method_decorator
+from rest_framework.authtoken.views import ObtainAuthToken
+from rest_framework.settings import api_settings
+
+from drf_yasg import openapi
+from drf_yasg.errors import SwaggerGenerationError
+from drf_yasg.generators import OpenAPISchemaGenerator
+from drf_yasg.utils import swagger_auto_schema
+
+
+def test_choice_field():
+    @method_decorator(name='post', decorator=swagger_auto_schema(
+        operation_description="Logins a user and returns a token",
+        manual_parameters=[
+            openapi.Parameter(
+                "username",
+                openapi.IN_FORM,
+                required=True,
+                type=openapi.TYPE_STRING,
+                description="Valid username or email for authentication"
+            ),
+            openapi.Parameter(
+                "password",
+                openapi.IN_FORM,
+                required=True,
+                type=openapi.TYPE_STRING,
+                description="Valid password for authentication",
+            ),
+        ]
+    ))
+    class CustomObtainAuthToken(ObtainAuthToken):
+        throttle_classes = api_settings.DEFAULT_THROTTLE_CLASSES
+
+    urlpatterns = [
+        url(r'token/$', CustomObtainAuthToken.as_view()),
+    ]
+
+    generator = OpenAPISchemaGenerator(
+        info=openapi.Info(title="Test generator", default_version="v1"),
+        patterns=urlpatterns
+    )
+
+    with pytest.raises(SwaggerGenerationError):
+        generator.get_schema(None, True)