Correction LDAP validation (#342)

* Correction LDAP username validation

As https://msdn.microsoft.com/en-us/library/aa366101(v=vs.85).aspx describe spaces should not be in start or at the end of username but they can be inside the username. So please check my solution for it.

* Check for zero length passwords in LDAP module.

According to https://tools.ietf.org/search/rfc4513#section-5.1.2 LDAP client should always check before bind whether a password is an empty value. There are at least one LDAP implementation which does not return error if you try to bind with DN set and empty password - AD.

* Clearing the login/email spaces at the [start/end]

models/login_source.go

@@ -548,9 +548,9 @@ func ExternalUserLogin(user *User, login, password string, source *LoginSource,
 func UserSignIn(username, password string) (*User, error) {
 	var user *User
 	if strings.Contains(username, "@") {
-		user = &User{Email: strings.ToLower(username)}
+		user = &User{Email: strings.ToLower(strings.TrimSpace(username))}
 	} else {
-		user = &User{LowerName: strings.ToLower(username)}
+		user = &User{LowerName: strings.ToLower(strings.TrimSpace(username))}
 	}
 
 	hasUser, err := x.Get(user)