Fix: Normalize running binary name detection (Windows)

This prevents invoking sendmail when the executed name differs from the actual binary name (eg: running `mailpit` instead of `mailpit.exe`). See #14

main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/axllent/mailpit/cmd"
 	sendmail "github.com/axllent/mailpit/sendmail/cmd"
@@ -15,10 +16,19 @@ func main() {
 	}
 
 	// running directly
-	if filepath.Base(exec) == filepath.Base(os.Args[0]) {
+	if normalize(filepath.Base(exec)) == normalize(filepath.Base(os.Args[0])) {
 		cmd.Execute()
 	} else {
 		// symlinked
 		sendmail.Run()
 	}
 }
+
+// Normalize returns a lowercase string stripped of the file extension (if exists).
+// Used for detecting Windows commands which ignores letter casing and `.exe`.
+// eg: "MaIlpIT.Exe" returns "mailpit"
+func normalize(s string) string {
+	s = strings.ToLower(s)
+
+	return strings.TrimSuffix(s, filepath.Ext(s))
+}