bugfix json_format.py

In the _RegularMessageToJsonObject method, there is no str type conversion for non-bytes type values in the repeated content. This causes an exception in the MessageToJson method, as the jsons data does not allow for the occurrence of byte array type property values.

bug eg(contains proto and python code)：

------common.proto------
message SafetyInfo{//
  repeated LoginDevice deviceList = 1;
}

message LoginDevice {
  optional string uuid = 1 [default = ""];
  optional string deviceName = 2 [default = ""];
  optional string deviceType = 3 [default = ""];
  required uint32 lastTime = 4;
}

------test code(python)------
from google.protobuf.json_format import MessageToJson

import common_pb2

pb = common_pb2.SafetyInfo()
pb_hex = "0a4e0a2039323833663530356533363332396338356638623866343832613561323061651211636d4a38426c307a4d20446576696365731a0f6950686f6e6520694f532031332e3720dc85a9b00628010a410a206233356161326632366236343966313536393466663761336263303434323163120950432dcdacd0cbb4ef1a0a57696e646f777320313020d5d2c6e705280010001800200028003001"
pb.ParseFromString(bytes.fromhex(pb_hex))
print(pb)
print(MessageToJson(pb))

python/google/protobuf/json_format.py

@@ -303,7 +303,7 @@ class _Printer(object):
         # Use base64 Data encoding for bytes
         return base64.b64encode(value).decode('utf-8')
       else:
-        return value
+        return str(value)
     elif field.cpp_type == descriptor.FieldDescriptor.CPPTYPE_BOOL:
       return bool(value)
     elif field.cpp_type in _INT64_TYPES: