Only format Unexpected::Float with decimal point if it is finite

[jamessan]

bef110b changed the display for unexpected floats to always append a
".0" if there was no decimal point found in the formatting of the float.

However, this should only be relevant for finite (i.e., not NaN or inf)
values. The change introduced a test failure in the ordered-float
crate due to this:

 ---- impl_serde::test_fail_on_nan stdout ----
 thread 'impl_serde::test_fail_on_nan' panicked at 'assertion failed: `(left == right)`
   left: `Error { msg: "invalid value: floating point `NaN.0`, expected float (but not NaN)" }`,
  right: `"invalid value: floating point `NaN`, expected float (but not NaN)"`', src/lib.rs:1554:9
 stack backtrace:
    0: rust_begin_unwind
              at /usr/src/rustc-1.70.0/library/std/src/panicking.rs:578:5
    1: core::panicking::panic_fmt
              at /usr/src/rustc-1.70.0/library/core/src/panicking.rs:67:14
    2: core::panicking::assert_failed_inner
    3: core::panicking::assert_failed
              at /usr/src/rustc-1.70.0/library/core/src/panicking.rs:228:5
    4: serde_test::assert::assert_de_tokens_error
              at /usr/share/cargo/registry/serde_test-1.0.171/src/assert.rs:228:19
    5: ordered_float::impl_serde::test_fail_on_nan
              at ./src/lib.rs:1554:9
    6: ordered_float::impl_serde::test_fail_on_nan::{{closure}}
              at ./src/lib.rs:1553:27
    7: core::ops::function::FnOnce::call_once
              at /usr/src/rustc-1.70.0/library/core/src/ops/function.rs:250:5
    8: core::ops::function::FnOnce::call_once
              at /usr/src/rustc-1.70.0/library/core/src/ops/function.rs:250:5
 note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[dtolnay]

Thanks!