Tables keys are not quoted

[x-hgg-x]

Describe the bug
Tables keys are not quoted.

To Reproduce

type sub struct {
	Num int
}
type s struct {
	SubField sub `toml:"%%%"`
}

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

Output:

[%%%]
  Num = 0

Expected behavior
Expected output:

["%%%"]
  Num = 0

Versions

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

Additional context
None.