Fix proposition for maxScore related notice + unit test (Issue #175)

This a bug fix proposition for Issue #175 initialy reported and fixed by @mcorne along with a unit test extension.

Here is an example of a test to reproduce the case

```
/select?q=id:466623&fl=id,score,title&mlt=true&mlt.fl=title => maxScore in the result
/select?q=id:466623&fl=id,score,title&mlt=true&mlt.fl=title&mlt.count=10 => maxScore not in the result
```