Response content is omitted if it is erroneous

## Long story short

Some http response content could be broken, in this case caller should be able to allow access to raw response content.

## Expected behaviour

Disable the gzip decompression on demand and return the raw data.

## Actual behaviour

Changes due to set env variable, see below.

## Steps to reproduce

```
import aiohttp
import asyncio
run = asyncio.get_event_loop().run_until_complete
resp = run(aiohttp.request('get', 'http://www.rofu.de'))
#FlowControlStreamReader protocol
run(resp.read())
#b''
```
when `AIOHTTP_NO_EXTENSIONS=1`

```
import aiohttp
import asyncio
run = asyncio.get_event_loop().run_until_complete
resp = run(aiohttp.request('get', 'http://www.rofu.de'))
#FlowControlStreamReader protocol
len(run(resp.read())) # blocks and raises TimeoutError
```

## Your environment

python 3.5.2
aiohttp 2.0.7
ubuntu 16.04