Fix fields and source_filter setup

[sothawo]

The current code erroneously uses thefields parameter of a query and adds them to the source_filter of a query. But fields and source_filter define different things. fields is used to return the values that are contained in the mapping or scripted or runtime fields whereas source_filter is used to return the specified the given entries from the _source of the document.

This needs to be fixed. This might break code that relies on fields being interpreted as source_filter included fields.