## Bug description

We've had a user create a funnel with Korean characters and 17 steps, which causes this issue:
![image](https://user-images.githubusercontent.com/1727427/120301818-8c212a00-c2cd-11eb-8b00-7ba242e4a9ab.png)

## Expected behavior

We should send requests to api/insight/funnel using POST instead of GET.

## How to reproduce

1.
2.
3.

## Environment

- [ ] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

## Additional context



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posthog/api/test/test_insight.py
import json
from datetime import timedelta
from django.test.utils import override_settings
from django.utils import timezone
from freezegun import freeze_time
from rest_framework import status
from posthog.ee import is_clickhouse_enabled
from posthog.models.dashboard_item import DashboardItem
from posthog.models.event import Event
from posthog.models.filters import Filter
from posthog.models.person import Person
from posthog.models.team import Team
from posthog.test.base import APIBaseTest
def insight_test_factory(event_factory, person_factory):
    class TestInsight(APIBaseTest):
        CLASS_DATA_LEVEL_SETUP = False
        def test_get_insight_items(self):