Cannot easily access max_field_size in client response

## Long story short

The http client doesn't really support overriding the default header value length of 8190, even though the parser parameter is there https://github.com/aio-libs/aiohttp/blob/e7c9390111932dd6dbb642170d7a0da1876271ec/aiohttp/http_parser.py#L59

This results in the error message:

```
aiohttp.client_exceptions.ClientResponseError: 400, message='Got more than 8190 bytes when reading Header value is too long.'
```

Stack trace from real app:
```

File "/usr/local/lib/python3.6/site-packages/pyportify/google.py", line 113, in _http_get
params=merged_params,
File "/usr/local/lib/python3.6/site-packages/aiohttp/helpers.py", line 97, in iter
ret = yield from self._coro
File "/usr/local/lib/python3.6/site-packages/aiohttp/client.py", line 241, in _request
yield from resp.start(conn, read_until_eof)
File "/usr/local/lib/python3.6/site-packages/aiohttp/client_reqrep.py", line 564, in start
message=exc.message, headers=exc.headers) from exc
aiohttp.client_exceptions.ClientResponseError: 400, message='Got more than 8190 bytes when reading Header value is too long.'
```

## Expected behaviour

I can either set the parser in the session/get request, or set the max_field_size.

## Actual behaviour

I can't do either

## Steps to reproduce
I believe this will do it:

```
import aiohttp
import asyncio

@asyncio.coroutine
def main():
with aiohttp.ClientSession() as session:
resp = yield from session.get('http://test.xr6.me')
print(len(resp.headers['X-TEST-HEADER']))
resp.close()

if __name__ == "__main__":
loop = asyncio.get_event_loop()
loop.run_until_complete(
asyncio.gather(main())
)
loop.close()
```

## Your environment

Ubuntu jessie (bash for windows, reproduces on others though)
python3.4 (also reproduces on others)