LDAP DN compare should not be case sensitive

According to the LDAP protocol request for comments (RFCs),
RDNs should be compared case-insensitively.

At the moment ou=Groups != ou=groups. This patch addresses
the case sensitivity in the RDNs.

dn.go

@@ -205,3 +205,9 @@ func (r *RelativeDN) hasAllAttributes(attrs []*AttributeTypeAndValue) bool {
 func (a *AttributeTypeAndValue) Equal(other *AttributeTypeAndValue) bool {
 	return strings.EqualFold(a.Type, other.Type) && a.Value == other.Value
 }
+
+// EqualFold returns true if the AttributeTypeAndValue is equivalent to the specified AttributeTypeAndValue
+// Case of the attribute type and value is not significant
+func (a *AttributeTypeAndValue) EqualFold(other *AttributeTypeAndValue) bool {
+	return strings.EqualFold(a.Type, other.Type) && strings.EqualFold(a.Type, other.Type)
+}