fix(string): fix deserialization of string with default value

[finger563]

If I have a struct with a std::string in it that has a default value, e.g.

struct MyStruct {
  std::string my_string = "default_value";
};

And I want to serialize an instance of that struct such as:

MyStruct my_struct {.my_string = "custom_value"};

It will serialize properly, containing only the length byte and the bytes for "custom_value".

However, when I deserialize it I will get a struct which contains a string:

"default_valuecustom_value"

This is because the deserialization creates a default instance of the struct (using T()), and then the string from_bytes(...) does a value += character for each byte in the stream.

This PR fixes that by clearing out the string when deserializing it, so that the deserialized struct's string will only contain

"custom_value"