Cannot mock Send traits

[asomers]

mock_derive cannot mock traits that are Send. Example:

    fn send() {
        #[mock]
        pub trait A {
            fn foo(&self);
        }

        let mock = MockA::new();
        let _ = Box::new(mock) as Box<A + Send>;
    }

Gives this error:

error[E0277]: `(dyn std::ops::FnMut() + 'static)` cannot be sent between threads safely
   --> src/t_mock_derive.rs:251:17
    |
251 |         let _ = Box::new(mock) as Box<A + Send>;
    |                 ^^^^^^^^^^^^^^ `(dyn std::ops::FnMut() + 'static)` cannot be sent between threads safely                                                    
    |
    = help: the trait `std::marker::Send` is not implemented for `(dyn std::ops::FnMut() + 'static)`
    = note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<(dyn std::ops::FnMut() + 'static)>`
    = note: required because it appears within the type `std::boxed::Box<(dyn std::ops::FnMut() + 'static)>`
    = note: required because it appears within the type `std::option::Option<std::boxed::Box<(dyn std::ops::FnMut() + 'static)>>`
    = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Mutex<std::option::Option<std::boxed::Box<(dyn std::ops::FnMut() + 'static)>>>`
    = note: required because it appears within the type `<t_mock_derive::t::MockDerive as TestSuite>::send::MockMethodForA<()>`
    = note: required because it appears within the type `std::option::Option<<t_mock_derive::t::MockDerive as TestSuite>::send::MockMethodForA<()>>`
    = note: required because it appears within the type `<t_mock_derive::t::MockDerive as TestSuite>::send::MockA`
    = note: required for the cast to the object type `dyn <t_mock_derive::t::MockDerive as TestSuite>::send::A + std::marker::Send`