Fix sorting on scripted date and boolean fields (elastic#9285)

Backports PR elastic#9261

**Commit 1:**
Fix sorting on scripted date and boolean fields

The elasticsearch API only [supports][1][2] sort scripts of type `number` and
`string`. Since dates need to be returned as millis since the epoch for
visualizations to work anyway, we can simply add a condition to send dates
as type number in the sort API. ES will cast booleans if we tell them
its a string, so we can add a similar condition there as well.

[1]: https://www.elastic.co/guide/en/elasticsearch/reference/5.0/search-request-sort.html#_script_based_sorting
[2]: https://github.com/elastic/elasticsearch/blob/aeb97ff41298e26b107a733837dfe17f123c0c9b/core/src/main/java/org/elasticsearch/search/sort/ScriptSortBuilder.java#L359

Fixes: elastic#9257

* Original sha: 0d5a824
* Authored by Matthew Bargar <[email protected]> on 2016-11-29T21:05:48Z

**Commit 2:**
Add helpful comment

* Original sha: 14dc396
* Authored by Matthew Bargar <[email protected]> on 2016-11-30T21:02:41Z

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

@@ -38,7 +38,7 @@ export default function normalizeSortRequest(config) {
           inline: indexField.script,
           lang: indexField.lang
         },
-        type: indexField.type,
+        type: castSortType(indexField.type),
         order: direction
       };
     } else {
@@ -56,3 +56,20 @@ export default function normalizeSortRequest(config) {
     return normalized;
   }
 };
+
+// The ES API only supports sort scripts of type 'number' and 'string'
+function castSortType(type) {
+  const typeCastings = {
+    number: 'number',
+    string: 'string',
+    date: 'number',
+    boolean: 'string'
+  };
+
+  const castedType = typeCastings[type];
+  if (!castedType) {
+    throw new Error(`Unsupported script sort type: ${type}`);
+  }
+
+  return castedType;
+}