Revert buffer size increase.

There are some internals to the usage of the buffer that cause any change to it's size in `MsgPack.h` to result in a segfault. More research is needed to figure out where exactly the buffer gets used and whether its usage can be replaced by `mpack_writer_init_growable` for mostly-automatic memory management.

source/mpackCPP/MsgPack.h

@@ -28,7 +28,7 @@
 class MsgPack {
 private:
  mpack_writer_t writer;
- char data[4096];
+ char data[1024];
 
  void _getJson(std::stringstream &s, mpack_reader_t &reader) const;