Grpc proxy fix

[Mityuha]

According to this issue there is an error while connecting over grpc-proxy

Traceback (most recent call last):                                                                                                       
  File "<stdin>", line 1, in <module>                                                                                                    
  File "/home/dmitry/projects/project/env/lib/python3.6/site-packages/etcd3/leases.py", line 24, in refresh                         
    return list(self.etcd_client.refresh_lease(self.id))                                                                                 
  File "/home/dmitry/projects/project/env/lib/python3.6/site-packages/etcd3/client.py", line 42, in handler                       
    _translate_exception(exc)                                                                                                            
  File "/home/dmitry/projects/project/env/lib/python3.6/site-packages/etcd3/client.py", line 39, in handler                         
    for data in f(*args, **kwargs):                                                                                                      
  File "/home/dmitry/projects/project/env/lib/python3.6/site-packages/etcd3/client.py", line 792, in refresh_lease                  
    metadata=self.metadata):                                                                                                             
  File "/home/dmitry/projects/project/env/lib/python3.6/site-packages/grpc/_channel.py", line 364, in __next__                      
    return self._next()                                                                                                                  
  File "/home/dmitry/projects/project/env/lib/python3.6/site-packages/grpc/_channel.py", line 358, in _next                         
    raise self                                                                                                                           
grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with:                                                                     
        status = StatusCode.CANCELLED                                                                                                    
        details = "context canceled"                                                                                                     
        debug_error_string = "{"created":"@1568820170.012790691","description":"Error received from peer ipv4:127.0.0.1:23790","file":"sr
c/core/lib/surface/call.cc","file_line":1052,"grpc_message":"context canceled","grpc_status":1}"

This patch based on that investigation. Fix allows requests that are proxied via gRPC to succeed as expected

[codecov-io]

Codecov Report

Merging #845 into master will decrease coverage by 0.11%.
The diff coverage is 92.1%.

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
- Coverage   94.02%   93.91%   -0.12%     
==========================================
  Files          10       10              
  Lines         787      822      +35     
==========================================
+ Hits          740      772      +32     
- Misses         47       50       +3

Impacted Files	Coverage Δ
etcd3/client.py	95.99% <92.1%> (-0.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cdc4c48...796cff9. Read the comment docs.