DatabaseCache backend raises TypeError if get/delete received key as integer.

[Hiroki Kiyohara]

After Django 2.2, DatabaseCache backend will raise TypeError when get/delete method received integer key.

>>> cache = caches["..."]
<django.core.cache.backends.db.DatabaseCache object at 0x7f2d5ce37ac8>
>>> cache.get(1)
Traceback (most recent call last):
  File "/usr/lib/python3.7/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<console>", line 1, in <module>
  File "/home/hirokiky/.../venv/lib/python3.7/site-packages/django/core/cache/backends/db.py", line 52, in get
    return self.get_many([key], version).get(key, default)
  File "/home/hirokiky/.../venv/lib/python3.7/site-packages/django/core/cache/backends/db.py", line 60, in get_many
    self.validate_key(key)
  File "/home/hirokiky/.../venv/lib/python3.7/site-packages/django/core/cache/backends/base.py", line 245, in validate_key
    if len(key) > MEMCACHE_MAX_KEY_LENGTH:

I know it's not a bug. Because we should pass key as string.

key should be a str, and value can be any picklable Python object.

​https://docs.djangoproject.com/en/2.2/topics/cache/#basic-usage

Before Django 2.2, key argument had been formatted by make_key method, and it would convert integer to string.
But now Django 2.2 will call validate_key before make_key, so it will raise TypeError if you pass integer key.

I think describing about this change on Django 2.2 release note is better
(as small backward incompatible change).

​https://docs.djangoproject.com/en/2.2/releases/2.2/

Note: This behaviour will happen after this optimization.
https://code.djangoproject.com/ticket/29584