create a failing test for #26

[softprops]

What did you implement:

Closes: #26

just a failing test for now to kick start this

How did you verify your change:

when this test passes. the issue will be fixed

What (if anything) would need to be called out in the CHANGELOG for the next release:

add support for serde flattened structs

[softprops]

current status

$ cargo -q test -- deserialize_flatten_from_iter                                                                                                ✘ 101 

running 1 test
test tests::deserialize_flatten_from_iter ... FAILED

failures:

---- tests::deserialize_flatten_from_iter stdout ----
thread 'tests::deserialize_flatten_from_iter' panicked at 'Custom(
    "invalid type: string \"1\", expected usize"
)', src/lib.rs:423:23
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.


failures:
    tests::deserialize_flatten_from_iter

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 8 filtered out