Marshaling is wrong when a key contains a dot

[x-hgg-x]

Describe the bug
Marshaling is wrong when a key contains a dot.

To Reproduce

type sub struct {
	Num int `toml:"c.d"`
}
type s struct {
	SubField sub `toml:"a.b"`
}

t, _ := toml.Marshal(s{sub{0}})
fmt.Println(string(t))

Output:

[a]
  [a.b]
    [a.b.c]
      d = 0

Expected behavior
Expected output:

["a.b"]
  "c.d" = 0

Versions

• go-toml: last version of master (9ccd9bb)
• go: 1.14
• operating system: Linux

Additional context
None.