Fix unpickle obj attribute error

[junnplus]

https://paste.ubuntu.com/25721297/
raise AttributeError when unpickle object get attribute

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-b4fa9c6bca01> in <module>()
----> 1 getattr(hongbao, 'detail_json')

AttributeError: 'THongbao' object has no attribute 'detail_json'

[codecov]

Codecov Report

Merging #34 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #34   +/-   ##
=======================================
  Coverage   80.31%   80.31%           
=======================================
  Files          35       35           
  Lines        3475     3475           
=======================================
  Hits         2791     2791           
  Misses        684      684

Impacted Files	Coverage Δ
thriftpy2/protocol/binary.py	90.27% <100%> (ø)	⬆️
thriftpy2/protocol/compact.py	80.39% <100%> (ø)	⬆️

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[ethe]

Please describe your problem in detail and how your patch fixes it. The code you post above is not a regular way to use Thriftpy.

[junnplus]

Steps to reproduce

1. define a thrift file

# foo.thrift
struct TRequest {
    1: optional i32 id,
    2: optional string name,
}

service FooService {
    bool api(1: TRequest request)
}

2. construct a request and serialize it with pickle, which will be pushed to the MQ

# client
import pickle

request = foo_thrift.Request(
    id=1,
    name='foo'
)

pickled_request = pickle.dumps(request)
message_queue.push(pickled_request)

3. before the message has been consumed, we announced a new field, and the new thrift struct looks like

struct TRequest {
    1: optional i32 id,
    2: optional string name,
    3: optional string bar,
}

4. the message consumer receive the message and used as param to call api

pickled_request = message_queue.pop()
request = pickle.loads(pickled_request)
foo_service.api(request)  // errors will occur

now we got a TDecodeException

Conclusion

because the message received from MQ does not have the field bar,
from here we know that each field defined in the thrift specification will be considered as required, which is not backward compatible,
when we serialize the request by new specification, the AttributeError will be raise

This PR provide a workaround to ensure the compatibility

[ethe]

Thanks.