Deduplicate data passed to FormPayload

[germsvel]

Closes #110

What changed?

Currently, FormData keeps the data passed as a list of tuples. That's not an optimal strategy since it doesn't deal with duplicates well.

That being said, there are two competing behaviors that we need to handle:

• One the one hand, we want to keep duplicates of list-like names (e.g. {"contact[]", "email"}.
• On the other hand, we don't really want to keep duplicates of the same operation done multiple times (e.g. [{"email", "[email protected]"}, {"email", "[email protected]"}]).

In the latter case, we can imagine a user doing a fill_in twice, where the email field changed twice. In the first case, we might have something like a checkbox where the user can check multiple values.

Like we said, a list of tuples doesn't seem to be the optimal data structure for FormData. But while we have it, we handle deduplication of keys in FormPayload.new/1.

But we have to consider one more case. If a user calls:

session
|> fill_in("Email", with: "[email protected]")
|> fill_in("Email", with: "[email protected]")
|> fill_in("Email", with: "[email protected]")

The final record should show [email protected] as the email. If we do a simple Enum.uniq/1, we dedup [email protected], leaving [email protected] as the last operation -- which is not what we want. Thus, our current implementation has to reverse the list of data, unique it, and then reverse it again.