Fix compatibility with aiohttp==3.7.0
Fix compatibility with aiohttp==3.7.0

The new version of aiohttp has made `limit` a required argument for the
StreamReader class. This change adds an explicit limit of 2 ** 16 which
is the same as what aiohttp uses internally.

Using `limit` as a keyword (rather than a positional argument) keeps
compatibility with previous versions of aiohttp.

Fixes #173

pnuckowski/aioresponses