Define a new Features() method for Fs

Optional interfaces are becoming more important in rclone,
--track-renames and --backup-dir both rely on them.

Up to this point rclone has used interface upgrades to define optional
behaviour on Fs objects.  However when one Fs object wraps another it
is very difficult for this scheme to work accurately.  rclone has
relied on specific error messages being returned when the interface
isn't supported - this is unsatisfactory because it means you have to
call the interface to see whether it is supported.

This change enables accurate detection of optional interfaces by use
of a Features struct as returned by an obligatory Fs.Features()
method.  The Features struct contains flags and function pointers
which can be tested against nil to see whether they can be used.

As a result crypt and hubic can accurately reflect the capabilities of
the underlying Fs they are wrapping.

googlecloudstorage/googlecloudstorage.go

@@ -130,10 +130,11 @@ func init() {
 // Fs represents a remote storage server
 type Fs struct {
 	name          string           // name of this remote
+	root          string           // the path we are working on if any
+	features      *fs.Features     // optional features
 	svc           *storage.Service // the connection to the storage server
 	client        *http.Client     // authorized client
 	bucket        string           // the bucket we are working on
-	root          string           // the path we are working on if any
 	projectNumber string           // used for finding buckets
 	objectACL     string           // used when creating new objects
 	bucketACL     string           // used when creating new buckets
@@ -175,6 +176,11 @@ func (f *Fs) String() string {
 	return fmt.Sprintf("Storage bucket %s path %s", f.bucket, f.root)
 }
 
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features {
+	return f.features
+}
+
 // Pattern to match a storage path
 var matcher = regexp.MustCompile(`^([^/]*)(.*)$`)
 
@@ -234,6 +240,7 @@ func NewFs(name, root string) (fs.Fs, error) {
 		objectACL:     fs.ConfigFileGet(name, "object_acl"),
 		bucketACL:     fs.ConfigFileGet(name, "bucket_acl"),
 	}
+	f.features = (&fs.Features{ReadMimeType: true, WriteMimeType: true}).Fill(f)
 	if f.objectACL == "" {
 		f.objectACL = "private"
 	}