Make == compare Dict values using == instead of isequal

After fc05d74d0bd2117c8691e68e751ee9b212a5d3a2:

```julia
julia> 1 == 1.0
true
julia> {"a"=>1} == {"a"=>1.0}
false
```

See also #4150. This fix is a little weird because of the asymmetry between keys and values (`==` implies that keys are `isequal` and values are `==`), but the current behavior is also a little weird.