fix(db): ensure dropDatabase always uses primary read preference

NODE-1378

lib/db.js

@@ -905,11 +905,10 @@ Db.prototype.dropDatabase = function(options, callback) {
   decorateWithWriteConcern(cmd, this, options);
 
   // Ensure primary only
-  const finalOptions = Object.assign(
-    {},
-    { readPreference: ReadPreference.PRIMARY },
-    this.s.options
-  );
+  const finalOptions = Object.assign({}, this.s.options, {
+    readPreference: ReadPreference.PRIMARY
+  });
+
   if (options.session) {
     finalOptions.session = options.session;
   }

test/unit/db_tests.js

@@ -0,0 +1,47 @@
+'use strict';
+
+const EventEmitter = require('events');
+const expect = require('chai').expect;
+const sinon = require('sinon');
+
+class MockTopology extends EventEmitter {
+  constructor() {
+    super();
+  }
+
+  capabilities() {
+    return {};
+  }
+}
+
+const test = {};
+describe('Database', function() {
+  before(() => {
+    // NOTE: These modules are being used prior to test run. In order to monkey-patch them
+    //       we must remove their cached versions.
+    const resolvedUtils = require.resolve('../../lib/utils');
+    const resolvedDb = require.resolve('../../lib/db');
+    delete require.cache[resolvedUtils];
+    delete require.cache[resolvedDb];
+    test.utils = require('../../lib/utils');
+
+    // create a sandbox for stub cleanup
+    test.sandbox = sinon.sandbox.create();
+  });
+
+  afterEach(() => test.sandbox.restore());
+
+  it('should ignore a readPreference for dropDatabase', {
+    metadata: { requires: { topology: 'single' } },
+    test: function() {
+      sinon.stub(test.utils, 'executeOperation').callsFake((topology, operation, args) => {
+        const options = args[args.length - 2];
+        expect(options.readPreference).to.equal('primary');
+      });
+
+      const Db = require('../../lib/db');
+      const db = new Db('fakeDb', new MockTopology(), { readPreference: 'nearest' });
+      db.dropDatabase();
+    }
+  });
+});