fix(mongodb+srv): respect overriding SRV-provided properties

This PR fixes the spec test and corrects the behavior of the TXT record
overriding the querystring by reversing the merge.

NODE-2335

Author: mpilar

lib/core/uri_parser.js

@@ -100,7 +100,7 @@ function parseSrvConnectionString(uri, options, callback) {
           );
         }
 
-        Object.assign(result.query, record);
+        result.query = Object.assign({}, record, result.query);
       }
 
       // Set completed options back into the URL object.

test/core/unit/mongodb_srv_tests.js

@@ -45,6 +45,10 @@ describe('mongodb+srv', function() {
                 expect(result.options.ssl).to.equal(test[1].options.ssl);
               }
 
+              if (test[1].options && test[1].options.authSource) {
+                expect(result.options.authsource).to.equal(test[1].options.authSource);
+              }
+
               if (
                 test[1].parsed_options &&
                 test[1].parsed_options.user &&