Doctests no longer build with default features

[tarcieri]

This was somehow introduced in #752 which bumped aead to v0.6.0-rc.4, but I am baffled as to how or what's even happening.

Example reproduction

$ cd aes-gcm
$ cargo test --doc
[...]
---- aes-gcm/src/lib.rs - (line 150) stdout ----
error[E0432]: unresolved import `aes_gcm::aead::Generate`
  --> aes-gcm/src/lib.rs:156:28
   |
8  |     aead::{Aead, AeadCore, Generate, Key, KeyInit},
   |                            ^^^^^^^^ no `Generate` in the root
   |
note: found an item that was configured out

Notes

This occurs in several crates, any of which use this style of gating:

#![cfg_attr(feature = "getrandom", doc = "```")]
#![cfg_attr(not(feature = "getrandom"), doc = "```ignore")]

Curiously, if the first line is changed to:

#![cfg_attr(feature = "getrandom", doc = "```ignore")]

Then the test is successfully ignored, suggesting this gating somehow isn't working correctly:

running 2 tests
test aes-gcm/src/lib.rs - (line 150) ... ignored
test aes-gcm/src/lib.rs - (line 189) ... ignored

Note the other example, which depends on both the arrayvec and getrandom features, is successfully being ignored with logic like:

#![cfg_attr(all(feature = "getrandom", feature = "arrayvec"), doc = "```")]
#![cfg_attr(
    not(all(feature = "getrandom", feature = "arrayvec")),
    doc = "```ignore"
)]

Perhaps the weirdest part is nothing in #752 actually changed how this gating worked, it just started behaving differently when the aead crate was upgraded from v0.6.0-rc.3 to v0.6.0-rc.4.