Inserts do not replace previous keys

```go
package main

import (
	"fmt"

	"github.com/vishalkuo/bimap"
)

func main() {
	m := bimap.NewBiMap()
	m.Insert("foo", 100)
	m.Insert("foo", 200)
	exists := m.ExistsInverse(100)
	fmt.Println(exists)
}
```

This prints true. Intuitively I would expect 200 to replace 100. If this is intentional behavior I feel it should be documented. Thanks for the project btw.