Fix []*Toml.Tree being wrapped in *Toml.Value (#110)

Nodes can be either *Toml.Tree, []*Toml.Tree, or *Toml.Value.
Arrays of trees were incorrectly wrapped in a *Toml.Value,
making the conversion functions think they were leaf nodes.

parser.go

@@ -211,7 +211,7 @@ func (p *tomlParser) parseAssign() tomlParserStateFn {
 	var toInsert interface{}
 
 	switch value.(type) {
-	case *TomlTree:
+	case *TomlTree, []*TomlTree:
 		toInsert = value
 	default:
 		toInsert = &tomlValue{value, key.Position}