Fix maps not preserving float -0.0.

[aaronjeline]

Maps currently don't preserve -0.0. If you add -0.0 as a value in a map, and read it, you will get 0.0.

This isn't true of other structures.

It happens because the default value is 0.0, and when adding a new value, the code checks if it is equal to the default value using PartialEq, which for floats treats 0.0 and -0.0 as equal.