Only attempt to change password if the confirmation field is filled in

Firefox autocompletes the password field (but not the password
confirmation field) for me. This makes it annoying to use the settings
page, because miniflux thinks I'm trying to change my password and
complains that the fields don't match.

ui/form/settings_test.go

@@ -0,0 +1,60 @@
+package form // import "miniflux.app/ui/form"
+
+import (
+	"testing"
+)
+
+func TestValid(t *testing.T) {
+	settings := &SettingsForm{
+		Username:       "user",
+		Password:       "hunter2",
+		Confirmation:   "hunter2",
+		Theme:          "default",
+		Language:       "en_US",
+		Timezone:       "UTC",
+		EntryDirection: "asc",
+	}
+
+	err := settings.Validate()
+	if err != nil {
+		t.Error(err)
+	}
+}
+
+func TestConfirmationEmpty(t *testing.T) {
+	settings := &SettingsForm{
+		Username:       "user",
+		Password:       "hunter2",
+		Confirmation:   "",
+		Theme:          "default",
+		Language:       "en_US",
+		Timezone:       "UTC",
+		EntryDirection: "asc",
+	}
+
+	err := settings.Validate()
+	if err != nil {
+		t.Error(err)
+	}
+
+	if settings.Password != "" {
+		t.Error("Password should have been cleared")
+	}
+}
+
+func TestConfirmationIncorrect(t *testing.T) {
+	settings := &SettingsForm{
+		Username:       "user",
+		Password:       "hunter2",
+		Confirmation:   "unter2",
+		Theme:          "default",
+		Language:       "en_US",
+		Timezone:       "UTC",
+		EntryDirection: "asc",
+	}
+
+	err := settings.Validate()
+	if err == nil {
+		t.Error("Validate should return an error")
+	}
+}