fix(mongos-replset): pass connect options to child server instances

This allows for options determined during uri parsing to make it
down to the creation of child Server instances using the legacy
SDAM topology classes.

NODE-1798

lib/topologies/mongos.js

@@ -286,7 +286,7 @@ Mongos.prototype.connect = function(options) {
   // Create server instances
   var servers = this.s.seedlist.map(function(x) {
     const server = new Server(
-      Object.assign({}, self.s.options, x, {
+      Object.assign({}, self.s.options, x, options, {
         authProviders: self.authProviders,
         reconnect: false,
         monitoring: false,

lib/topologies/replset.js

@@ -960,12 +960,14 @@ ReplSet.prototype.connect = function(options) {
   var self = this;
   // Add any connect level options to the internal state
   this.s.connectOptions = options || {};
+
   // Set connecting state
   stateTransition(this, CONNECTING);
+
   // Create server instances
   var servers = this.s.seedlist.map(function(x) {
     return new Server(
-      Object.assign({}, self.s.options, x, {
+      Object.assign({}, self.s.options, x, options, {
         authProviders: self.authProviders,
         reconnect: false,
         monitoring: false,

test/tests/unit/replset/compression_tests.js

@@ -0,0 +1,53 @@
+'use strict';
+
+const ReplSet = require('../../../../lib/topologies/replset');
+const mock = require('mongodb-mock-server');
+const ReplSetFixture = require('../common').ReplSetFixture;
+const expect = require('chai').expect;
+
+describe('Compression (ReplSet)', function() {
+  let test;
+  before(() => (test = new ReplSetFixture()));
+  afterEach(() => mock.cleanup());
+  beforeEach(() => test.setup());
+
+  it('should pass compression information to child server instances on connect', function(done) {
+    const compressionData = [];
+    test.primaryServer.setMessageHandler(request => {
+      const doc = request.document;
+      if (doc.ismaster) {
+        compressionData.push(doc.compression);
+        request.reply(test.primaryStates[0]);
+      }
+    });
+
+    test.firstSecondaryServer.setMessageHandler(request => {
+      const doc = request.document;
+      if (doc.ismaster) {
+        compressionData.push(doc.compression);
+        request.reply(test.firstSecondaryStates[0]);
+      }
+    });
+
+    const replSet = new ReplSet(
+      [test.primaryServer.address(), test.firstSecondaryServer.address()],
+      {
+        setName: 'rs',
+        haInterval: 10000,
+        connectionTimeout: 3000,
+        secondaryOnlyConnectionAllowed: true,
+        size: 1
+      }
+    );
+
+    replSet.on('fullsetup', () => {
+      compressionData.forEach(data => {
+        expect(data).to.eql(['zlib']);
+      });
+
+      done();
+    });
+
+    replSet.connect({ compression: { compressors: ['zlib'] } });
+  });
+});