fix: Prefix when serializing nested struct

[LauH1987]

The current implementation doesn't "pop" the prefix correctly when finishing serializing a nested struct. This leaves the prefix from the nested struct lingering and adds it to any field that is being serialized afterwards. This happens because ser.prefix_before.clone() is called after the serialization of the nested struct has already changed the prefix_before.

I've added some tests to show the behavior. Here's an example of one of the tests running on the current main branch. The serialized struct looks like:

Outer {
    a: 1,
    inner: Inner { x: 2 },
    b: 3,
}

running 1 test
test ser::tests::serialize_to_string_struct_field_after_nested ... FAILED

failures:

---- ser::tests::serialize_to_string_struct_field_after_nested stdout ----

thread 'ser::tests::serialize_to_string_struct_field_after_nested' (87451) panicked at src/ser.rs:660:9:
assertion `left == right` failed
  left: "A=1\nINNER_X=2\nB=3"
 right: "A=1\nINNER_X=2\nINNER_B=3"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    ser::tests::serialize_to_string_struct_field_after_nested

As showed b gets serialized with the INNER_ prefix.

[lucagoslar]

Thank you!