Make message.to_dict() return bytes for DATA type field (#386)

* change to_dict return bytes for DATA type

* reformat

* reformat

* add includes .h

---------

Co-authored-by: Brian Xu <brian.xu1@bytedance.com>

capnp/lib/capnp.pyx

@@ -1060,6 +1060,12 @@ cdef _to_dict(msg, bint verbose, bint ordered, bint encode_bytes_as_base64=False
         # encode the message as base64 and return utf-8 string
         return base64.b64encode(msg).decode('utf-8')
 
+    if msg_type is memoryview:
+        if encode_bytes_as_base64:
+            return base64.b64encode(bytes(msg)).decode('utf-8')
+        else:
+            return bytes(msg)
+
     return msg