drive: fix imports of text files

Before this change text file imports were ignored.  This was because
the mime type wasn't matched.

Fix this by adjusting the keys in the mime type maps as well as the
values.

See: https://forum.rclone.org/t/how-to-upload-text-files-to-google-drive-as-google-docs/9014

backend/drive/drive.go

@@ -676,28 +676,33 @@ func isPowerOfTwo(x int64) bool {
 }
 
 // add a charset parameter to all text/* MIME types
-func fixMimeType(mimeType string) string {
-	mediaType, param, err := mime.ParseMediaType(mimeType)
+func fixMimeType(mimeTypeIn string) string {
+	if mimeTypeIn == "" {
+		return ""
+	}
+	mediaType, param, err := mime.ParseMediaType(mimeTypeIn)
 	if err != nil {
-		return mimeType
+		return mimeTypeIn
 	}
-	if strings.HasPrefix(mimeType, "text/") && param["charset"] == "" {
+	mimeTypeOut := mimeTypeIn
+	if strings.HasPrefix(mediaType, "text/") && param["charset"] == "" {
 		param["charset"] = "utf-8"
-		mimeType = mime.FormatMediaType(mediaType, param)
+		mimeTypeOut = mime.FormatMediaType(mediaType, param)
 	}
-	return mimeType
+	if mimeTypeOut == "" {
+		panic(errors.Errorf("unable to fix MIME type %q", mimeTypeIn))
+	}
+	return mimeTypeOut
 }
-func fixMimeTypeMap(m map[string][]string) map[string][]string {
-	for _, v := range m {
+func fixMimeTypeMap(in map[string][]string) (out map[string][]string) {
+	out = make(map[string][]string, len(in))
+	for k, v := range in {
 		for i, mt := range v {
-			fixed := fixMimeType(mt)
-			if fixed == "" {
-				panic(errors.Errorf("unable to fix MIME type %q", mt))
-			}
-			v[i] = fixed
+			v[i] = fixMimeType(mt)
 		}
+		out[fixMimeType(k)] = v
 	}
-	return m
+	return out
 }
 func isInternalMimeType(mimeType string) bool {
 	return strings.HasPrefix(mimeType, "application/vnd.google-apps.")