Add test for merging of grouped results.

It fails for now because it is not implemented yet.
Add "key_column" and "merge_columns" options for search reducers
Don't add options for merging if not needed
Add "key_column" option for sort reducers
Fix expected cases for key_column
Revert wrong changes around request query's attributes
Update test for merging more strctly
Merge column value of records which have same key.

However, merged results are not sorted yet.
Sort unified items
Skip to sort if nothing is unified
Indent
Use "unify" instead of "merge" about column values
Don't add "_key" attribute to the output if it is not needed
Unify only grouped records
Unify any grouped results
Fix mismatched variable name
Update expected distributed search queries for non-grouped queries
Add test about search distributor for queries with "groupBy"