test: send content-type with proxy POST tests (elastic#10903)

Elasticsearch now requires that content-type be sent on all requests
with payloads, so our tests should be sending it with requests as well.

src/core_plugins/elasticsearch/lib/__tests__/routes.js

@@ -93,18 +93,27 @@ describe('plugins/elasticsearch', function () {
     testRoute({
       method: 'POST',
       url: '/elasticsearch/.kibana/__kibanaQueryValidator/_validate/query?explain=true&ignore_unavailable=true',
+      headers: {
+        'content-type': 'application/json'
+      },
       payload: { query: { query_string: { analyze_wildcard: true, query: '*' } } }
     });
 
     testRoute({
       method: 'POST',
       url: '/elasticsearch/_mget',
+      headers: {
+        'content-type': 'application/json'
+      },
       payload: { docs: [{ _index: '.kibana', _type: 'index-pattern', _id: '[logstash-]YYYY.MM.DD' }] }
     });
 
     testRoute({
       method: 'POST',
       url: '/elasticsearch/_msearch',
+      headers: {
+        'content-type': 'application/json'
+      },
       payload: '{"index":"logstash-2015.04.21","ignore_unavailable":true}\n{"size":500,"sort":{"@timestamp":"desc"},"query":{"bool":{"must":[{"query_string":{"analyze_wildcard":true,"query":"*"}},{"bool":{"must":[{"range":{"@timestamp":{"gte":1429577068175,"lte":1429577968175}}}],"must_not":[]}}],"must_not":[]}},"highlight":{"pre_tags":["@kibana-highlighted-field@"],"post_tags":["@/kibana-highlighted-field@"],"fields":{"*":{}}},"aggs":{"2":{"date_histogram":{"field":"@timestamp","interval":"30s","min_doc_count":0,"extended_bounds":{"min":1429577068175,"max":1429577968175}}}},"stored_fields":["*"],"_source": true,"script_fields":{},"docvalue_fields":["timestamp_offset","@timestamp","utc_time"]}\n' // eslint-disable-line max-len
     });