Restore _source fetching behavior

Kibana used to always include a `fields` parameter on Discover search
requests which included '_source' so that ES would return the _source
field for each hit. When ES removed the `fields` param we attempted to
switch to a new request body param `_source: true` to maintain the
same behavior. However, we missed one spot in the code that needed
updating in order to pass that param along to the actual request json.

This problem wasn't immediately obvious because _source is included by
default. However, this fixes a related issue where _source fields were
missing in Discover if any scripted fields existed. The presence of the
`script_fields` param in the request would disabled the automatic return
of _source. Now that we're correctly passing `_source: true`, _source
fetching works correctly even when scripted fields are present.

Fixes elastic#7699


Former-commit-id: 1cd6eec

src/ui/public/courier/data_source/_abstract.js

@@ -284,7 +284,8 @@ export default function SourceAbstractFactory(Private, Promise, PromiseEmitter)
 
         if (flatState.body.size > 0) {
           let computedFields = flatState.index.getComputedFields();
-          flatState.body.fields = computedFields.fields;
+          flatState.body.stored_fields = computedFields.storedFields;
+          flatState.body._source = computedFields._source;
           flatState.body.script_fields = flatState.body.script_fields || {};
           flatState.body.fielddata_fields = flatState.body.fielddata_fields || [];