browse: when sorting by size, sort directory section by name

Previously directories have been merely pulled to the front, and then
sorted arbitrarily. That is, their order among themselves depended on
the filesystem implementations. Something opaque to the visitor.

This fixes said inconsistency, and implements the by-size-then-by-name
order I initially intended for this.

caddyhttp/browse/browse.go

@@ -154,12 +154,21 @@ func (l bySize) Swap(i, j int) { l.Items[i], l.Items[j] = l.Items[j], l.Items[i]
 const directoryOffset = -1 << 31 // = math.MinInt32
 func (l bySize) Less(i, j int) bool {
 	iSize, jSize := l.Items[i].Size, l.Items[j].Size
+
+	// Directory sizes depend on the filesystem implementation,
+	// which is opaque to a visitor, and should indeed does not change if the operator choses to change the fs.
+	// For a consistent user experience directories are pulled to the front…
 	if l.Items[i].IsDir {
-		iSize = directoryOffset + iSize
+		iSize = directoryOffset
 	}
 	if l.Items[j].IsDir {
-		jSize = directoryOffset + jSize
+		jSize = directoryOffset
 	}
+	// … and sorted by name.
+	if l.Items[i].IsDir && l.Items[j].IsDir {
+		return strings.ToLower(l.Items[i].Name) < strings.ToLower(l.Items[j].Name)
+	}
+
 	return iSize < jSize
 }