Fix unexpected data length in aio buffer transport

[ethe]

No description provided.

[aiudirog]

It passes my tests with one change: rest_len = len(ret) - sz should be rest_len = sz - len(ret) since sz will be bigger.

However, if you use max for the read length, TAsyncSocket can't use readexactly otherwise we're back to the timeout errors. This means TAsyncSocket has to be wrapped in the buffered transport and couldn't be used as transport on its own because it's read method doesn't guarantee the number of bytes. If that's okay, then this PR should be it.

[ethe]

Yes, we should use read rather than readexactly here.

[aiudirog]

Yeah, looking at TSocket it does the same thing. Sounds good! I'll close the other PR.

[codecov]

Codecov Report

Merging #102 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   80.87%   80.91%   +0.03%     
==========================================
  Files          35       35              
  Lines        3529     3536       +7     
==========================================
+ Hits         2854     2861       +7     
  Misses        675      675

Impacted Files	Coverage Δ
thriftpy2/contrib/aio/transport/buffered.py	94% <100%> (+0.38%)	⬆️
thriftpy2/transport/buffered/__init__.py	95.12% <100%> (+0.52%)	⬆️

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