Fix issue that nothing is in 'Time field' dropdown when defining monitors by visual graph

[ftianli-amzn]

Issue #, if available:

In the console:

Alerting - ElasticsearchService - getMappings: { TypeError: Body can not be sent with method "GET"
    at Transport.request (/Users/ftianli/Github/kibana_7.9.1/node_modules/elasticsearch/src/lib/transport.js:215:7)
    at exec (/Users/ftianli/Github/kibana_7.9.1/node_modules/elasticsearch/src/lib/client_action.js:403:20)
    at ClientNamespace.call [as getMapping] (/Users/ftianli/Github/kibana_7.9.1/node_modules/elasticsearch/src/lib/client_action.js:61:16)
    at Promise (/Users/ftianli/Github/kibana_7.9.1/src/core/server/elasticsearch/legacy/cluster_client.ts:66:27)
    at new Promise (<anonymous>)
    at callAPI (/Users/ftianli/Github/kibana_7.9.1/src/core/server/elasticsearch/legacy/cluster_client.ts:65:18)
    at LegacyScopedClusterClient.scopedAPICaller (/Users/ftianli/Github/kibana_7.9.1/src/core/server/elasticsearch/legacy/cluster_client.ts:239:18)
    at LegacyScopedClusterClient.callAsCurrentUser (/Users/ftianli/Github/kibana_7.9.1/src/core/server/elasticsearch/legacy/scoped_cluster_client.ts:115:17)
    at handler (/Users/ftianli/Github/kibana_7.9.1/plugins/alerting-kibana-plugin/server/services/ElasticsearchService.js:112:30)
    at /Users/ftianli/Github/kibana_7.9.1/src/core/utils/context.ts:248:14
    at process._tickCallback (internal/process/next_tick.js:68:7) body: undefined, status: undefined }

Description of changes:
The body in the HTTP requests of the API calls of get alias, get indices and get mappings were not organized correctly during the migration to new Kibana platform (#209)

It caused nothing shows up in the 'Time field' dropdown menu when defining monitors by visual graph.

• Organize the request body as an object to make it have got correct name-value pairs
• Make the validation of the http requests more accurate, but it's not mandatory
• Fix a typo in a handler of the http request

With the changes in the PR, the "time field" can be read and selected:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.