Merge #130: Serialize msg_header using CBlockHeader instead of CBlock

3a9a162 Serialize msg_header using CBlockHeader instead of CBlock (str4d)

Pull request description:

  This resolves an issue I uncovered while updating `pynode` to work with `python-bitcoinlib` 0.7.0. The following usage was previously allowable, because `CBlock` was mutable:

  ```py
  db_block = self.chaindb.getblock(blkhash)
  block = copy.copy(db_block)
  block.vtx = []
  msg.headers.append(block)
  ```

  I [updated it](https://github.com/jgarzik/pynode/pull/19/files#diff-97b3827045a89ffba8170a14179aeb74L373) to the following, which resulted in the type error that this PR fixes:

  ```py
  block = self.chaindb.getblock(blkhash)
  msg.headers.append(block.get_header())
  ```

Tree-SHA512: ef324cd0baf04cb20a0e4d06425542ee86238527500544a8d1fde15ed2f91c138b536b90123ce6b19e43649a57ebc129cba72230801f1e96ab1f487717df158e

Author: petertodd

bitcoin/messages.py

@@ -336,11 +336,11 @@ def __init__(self, protover=PROTO_VERSION):
     @classmethod
     def msg_deser(cls, f, protover=PROTO_VERSION):
         c = cls()
-        c.headers = VectorSerializer.stream_deserialize(CBlock, f)
+        c.headers = VectorSerializer.stream_deserialize(CBlockHeader, f)
         return c
 
     def msg_ser(self, f):
-        VectorSerializer.stream_serialize(CBlock, self.headers, f)
+        VectorSerializer.stream_serialize(CBlockHeader, self.headers, f)
 
     def __repr__(self):
         return "msg_headers(headers=%s)" % (repr(self.headers))