Added protocol validation

lib/Connection.js

@@ -6,7 +6,7 @@ const https = require('https')
 const debug = require('debug')('elasticsearch')
 const decompressResponse = require('decompress-response')
 const pump = require('pump')
-const { TimeoutError } = require('./errors')
+const { TimeoutError, ConfigurationError } = require('./errors')
 
 class Connection {
   constructor (opts = {}) {
@@ -21,6 +21,10 @@ class Connection {
     this._status = opts.status || Connection.statuses.ALIVE
     this.roles = opts.roles || defaultRoles
 
+    if (!['http:', 'https:'].includes(this.url.protocol)) {
+      throw new ConfigurationError(`Invalid protocol: '${this.url.protocol}'`)
+    }
+
     const agentOptions = Object.assign({}, {
       keepAlive: true,
       keepAliveMsecs: 1000,