Prevent potential breakage through `form_urlencoded`

[PhilippGackstatter]

Hello,

we would like to point out a potential issue that you might want to prevent. The did-key crate depends on did_url which in turn depends on form_urlencoded. The latter crate is considering changes in their default-features which would break users of did_url. We assume that the did_url crate won't be changed to fix this issue, in particular that this PR won't be merged, so we're pointing it out to you instead. A fix on your side would be to add form_urlencoded to your dependencies and pin it, i.e.:

form_urlencoded = { version = "=1.1.0" }

We recently released a similar fix for reference.

Best regards.