Double underscores test fails with wrappers forced

[adetaylor]

Using the branch in #1250, test_double_underscores_ignored fails:

RUST_LOG=autocxx_engine=info AUTOCXX_FORCE_WRAPPER_GENERATION=1  cargo test test_double_underscores_ignored -- --nocapture

error[E0428]: the name `__default` is defined multiple times
 --> /var/folders/3w/3pyz6m7s1jn5dlm38v01w54h0000gp/T/.tmp0yAyMF/autocxx-ffi-default-gen.rs:1:2036
  |
1 | ..."] pub struct __default ; # [doc = "autocxx bindings couldn't be generated: Names containing __ are reserved by C++ so not acceptable to cxx"] pub struct __destructor ; # [doc = "autocxx bindings couldn't be generated: Names containing __ are reserved by C++ so not acceptable to cxx"] pub struct __copy ; # [doc = "autocxx bindings couldn't be generated: Names containing __ are reserved by C++ so not acceptable to cxx"] pub struct __copy_operator ; # [doc = "autocxx bindings couldn't be generated: Names containing __ are reserved by C++ so not acceptable to cxx"] pub struct __move ; # [doc = "autocxx bindings couldn't be generated: Names containing __ are reserved by C++ so not acceptable to cxx"] pub struct __move_operator ; # [doc = "autocxx bindings couldn't be generated: Names containing __ are reserved by C++ so not acceptable to cxx"] pub struct __default ; # ...
  |       ---------------------- previous definition of the type `__default` here