Improve document ranking when a query word is know to be exact

[Kerollmops]

Currently the documents ranking doesn't seems to take into account the fact that a query word is at the end of the query or not. When a word is at the middle of a query or is not followed by a space this word is more likely to be finished/in its exact form.

For example if a user query "cat and dog" we must understand that the word "cat" is a finished word and documents containing the exact representation must be more interresting than documents contaning words starting with "cat" like "catalog" "catherine"...

This is already one of the criterion that is used by default but it is nearly the last criterion in the list (link to the code).

[Kerollmops]

f05af4b