[whitespace] Fix spaces in `std.math.FloatingPointControl` example and unittest.
Fix `std.math.FloatingPointControl` two issues.

1. Do not call `setControlState` in destructor if not initialized (previously results in enabling all exceptions).
2. Call `initialize` on setting rounding mode (previously not called).
[docs] Improve `std.math.FloatingPointControl` example.
Merge pull request #1208 from denis-sh/std.math.FloatingPointControl-fixes

`std.math.FloatingPointControl` fixes