If a get field mapping request is issued, and all but the field can be
found, the response should return an empty JSON object instead of a 404.
Closes#4738
The get field mapping API now includes a mappings element after the index in its JSON
Added more consistent endpoint /{index}/_mapping/{type}/field/{fields}
and added endpoint /_mapping/{type}/field/{fields}
which are also used in tests
Added rest spec tests for wildcards and _all
Relates #4071
NOTE: This is not yet complete for 1.0. We need to return an empty JSON document instead
of a 404 if the field of an existing index and type is not found. However this is not
possible with the current data structure being returned. Needs to be finished for 1.0.
Alexander Reelsen
Honza Král
Boaz Leskes
Karel Minarik
Clinton Gormley