Fix size option for ldap.search method

This PR fixes issue https://github.com/ruby-ldap/ruby-net-ldap/issues/75. 

### Problem

When I search for ldap users and use `size` option I got `nil` result if amount of users in LDAP more than number provided in `size` options. So if you want to get first 50 LDAP users you get nil because its more than 50 users :) 

Describing problem in more simple way it looks like this for me:

```
Given I visit a shop to buy some beer
When I ask cashier to sell me 1 bottle of beer
Then he responds me: Size limit exceeded. 1297 beers available, 1 beer requested
And I return home with no beer
```

### Solution

Just check for `4` result_code from LDAP and return result in this case too. 


### Possible improvemens

I dont like this `0` and `4` magic numbers but I afraid to touch code unrelated to my fix. If you ask I can add `4` as constant like `LDAP_SIZE_LIMIT_EXCEEDED`

- - -

Thank you!!! 
I'd like to use a constant in this case. I didn't see one already defined, though, but I'd be in favor of one (or a few) for this.