Fixes Issue 914

[dyedgreen]

Closes #914

This fixes the issue raised and adds a test to prevent future regressions.

Note that this slightly changes to public api of juniper::Object's: add_field will now check if a new field exists and is an object, and in that case merge the objects keys instead of replacing the old object.

It might also make sense to not return anything from that function (which is a breaking change of the public interface, but seems to compile without modification to the existing uses in the juniper crate, I think the behavior was originally copied from std::collections::HashMap).

[dyedgreen]

(The CI fail seems to be related to rocket; but I'm not sure if that was caused by my change 😬)

(If I had to guess, there might be a breaking update of some dependencies with rocket:)

    |
616 |         rocket::ignite().manage(Database::new()).manage(Schema::new(
    |                 ^^^^^^ not found in `rocket`

[LegNeato]

Thank you for the fix! 🍻 The breakage is indeed some Rocket changes that have broken us.