Bug fix: CDataStream::GetAndClear() when nReadPos > 0

Changed CDataStream::GetAndClear() to use the most obvious
get get and clear instead of a tricky swap().

Added a unit test for CDataStream insert/erase/GetAndClear.

Note: GetAndClear() is not performance critical, it is used only
by the send-a-message-to-the-network code. Bug was not noticed
before now because the send-a-message code never erased from the
stream.

src/serialize.h

@@ -1104,8 +1104,8 @@ public:
     }
 
     void GetAndClear(CSerializeData &data) {
-        vch.swap(data);
-        CSerializeData().swap(vch);
+        data.insert(data.end(), begin(), end());
+        clear();
     }
 };