Rework tree from map (#139)

* Make TreeFromMap reflect to construct tree
* Fix wording of invalid value type in writeTo

Fixes #138, #139, #134 

⚠️ TreeFromMap signature changed to `TreeFromMap(map[string]interface{}) (*TomlTree, error)`

toml.go

@@ -28,10 +28,12 @@ func newTomlTree() *TomlTree {
 }
 
 // TreeFromMap initializes a new TomlTree object using the given map.
-func TreeFromMap(m map[string]interface{}) *TomlTree {
-	return &TomlTree{
-		values: m,
+func TreeFromMap(m map[string]interface{}) (*TomlTree, error) {
+	result, err := toTree(m)
+	if err != nil {
+		return nil, err
 	}
+	return result.(*TomlTree), nil
 }
 
 // Has returns a boolean indicating if the given key exists.