python-ecosys/aiohttp: Fix partial reads.

[FuNK3Y]

This one has been inspired by #1012 and should solve it

This problem does not only affect websockets. I experienced similar troubles when reading a JSON payload of ~6.5k from an ESP32.

[Carglglz]

I think it may be better to use readexactly stream method? e.g.

 async def read(self, sz=-1):
+        if sz > 0:
+            return self._decode(await self.content.readexactly(sz))
         return self._decode(await self.content.read(sz))

[FuNK3Y]

Hooo yes ! This is will clearly make the whole module more robust. I did the changes and did some testing and everything looks good so far

[dpgeorge]

Looks good, thanks.