fixe nginx location rule for _mapping

If you use a nginx proxy and elasticsearch with log4j you can see the following error message:

```
Error No index found at http://my.domain.local:80/logstash-2014.02.26/_mapping/field/*. Please create at least one index.If you're using a proxy ensure it is configured correctly.
```
Thats because the strict nginx location mapping rule.
Merge pull request #996 from motortalk/master

fixe nginx location rule for _mapping
Closes #1013