Patch to use schema ldif file

## Problem
When you add a schema ldif fill under 
>grails-app/ldap-server/<name>/schema

you get a exception
>The global schema subentry cannot be added since it exists by default.

## Cause: 
t's because the parsed ldif is used to do an add operation.
```java
    private consumeLdifReader(ldifReader) {
        while (ldifReader.hasNext()) {
            def entry = ldifReader.next()
            def ldif = LdifUtils.convertToLdif(entry, Integer.MAX_VALUE)
            directoryService.adminSession.add(directoryService.newEntry(ldif, entry.dn.toString()))
        }
    }
```

## Fix
You have to modify cn=schema because it exist and use CoreSession.modify(..) instead of add(..)

```java
	private consumeLdifReader(ldifReader) {
		while (ldifReader.hasNext()) {
			LdifEntry entry = ldifReader.next()
			if ( entry.isChangeModify() ) {
				directoryService.adminSession.modify(entry.dn, entry.modificationList)
			} else {
				def ldif = LdifUtils.convertToLdif(entry, Integer.MAX_VALUE)
				directoryService.adminSession.add(directoryService.newEntry(ldif, entry.dn.toString()))
			}
		}
	}
```

## Unit test
```java
	void testLoadSchemaLdif() {
		d2LdapServer.loadLdif("""
dn: cn=schema
changetype: modify
add: attributetypes
attributetypes: ( 1.3.6.1.4.1.99999.2.1.0.0 NAME 'myRegionalOffice' EQUALITY caseIgnoreMatch SUBSTR caseIgnoreSubstringsMatch SYNTAX '1.3.6.1.4.1.1466.115.121.1.15' )
-
add: objectclasses
objectclasses: (  1.3.6.1.4.1.99999.2.3.1.0.0  NAME 'myUser' SUP 'person' STRUCTURAL MAY ( myRegionalOffice ) )
-
""")
		d2LdapServer.loadLdif("""
dn: cn=cn3,dc=d2
cn: cn3
sn: sn
objectClass: person
objectClass: top
objectClass: myUser
myRegionalOffice: nowhere
""")
		assertTrue(d2LdapServer.exists("cn=cn3,dc=d2"))
	}
```