Fix: response content is empty when using cache

[yanxurui]

I am using this library in a project and found there is an issue when combined with cache.

Check the example below:

import asyncio
import aiohttp
from aiohttp_client_cache import CachedSession, CacheBackend


async def main():
    request_cache = CacheBackend(expire_after=60*60)
    async with CachedSession(cache=request_cache, auto_decompress=False) as s:
    # async with aiohttp.ClientSession() as s:
        while True:
            resp = await s.request('GET', 'https://www.googleapis.com/oauth2/v1/certs')
            content = await resp.content.read()
            body = await resp.read()
            import pdb
            pdb.set_trace()

# call main() in a loop
loop = asyncio.get_event_loop()
loop.run_until_complete(main())

In the 3rd iteration of the loop above, content will be an empty string while body is expected. This is probably because the content stream was GCed. Use .read() instead of .content.read() can fix the problem.

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