Allow float comparison with zero

cargo clippy -V: clippy 0.0.212 (d61b254 2019-02-19)

Clippy should allow float comparison with zero. The current behavior seems to be a regression as this has apparently already been reported and fixed in #165/#1142.

error: strict comparison of f32 or f64
   --> tests/test_datetime.rs:125:5
    |
125 |     assert_eq!(offset, 0f32);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: #[deny(clippy::float_cmp)] on by default
note: std::f32::EPSILON and std::f64::EPSILON are available.
   --> tests/test_datetime.rs:125:5
    |
125 |     assert_eq!(offset, 0f32);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#float_cmp
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

Start with tests/test_datetime.rs and reproduce the diagnostic with cargo clippy, then trace the float_cmp lint behavior. Done means the shown comparison with zero no longer emits the error, with regression coverage for the corrected behavior.