SetHeaderColSpans use with many headers bug

I don't know if I'm using `SetHeaderColSpans` correctly, but the layout is off:

Source ([playground](https://go.dev/play/p/zq3MMASfoSG)):

```go
package main

import (
"os"

"github.com/aquasecurity/table"
)

func main() {
t := table.New(os.Stdout)
t.SetHeaderColSpans(0, 2)
t.SetHeaderColSpans(1, 2)
t.AddHeaders("Header 1")
t.AddHeaders("Header 2")
t.AddHeaders("Header 3", "Header 4")
t.AddRow("Col 1", "Col 2")
t.AddFooters("Footer 1", "Footer 2")
t.Render()
}

```

Outputs:

```sh
┌─────────────────────┐
│ Header 1 │
├────────────────────────┤
│ Header 2 │
├──────────┬──────────┤
│ Header 3 │ Header 4 │
├──────────┼──────────┤
│ Col 1 │ Col 2 │
├──────────┼──────────┤
│ Footer 1 │ Footer 2 │
└──────────┴──────────┘
```

Expected:

```sh
┌─────────────────────┐
│ Header 1 │
├─────────────────────┤
│ Header 2 │
├──────────┬──────────┤
│ Header 3 │ Header 4 │
├──────────┼──────────┤
│ Col 1 │ Col 2 │
├──────────┼──────────┤
│ Footer 1 │ Footer 2 │
└──────────┴──────────┘
```

Start by running the linked Go playground reproduction and trace the SetHeaderColSpans entry point through table rendering. The fix is done when multiple spanned headers produce borders aligned with the two-column rows and a regression test covers the expected output.

Written by the indexing model from the issue text.