JAVA-2271: Ensure that ConnectionString.getCredentialList returns an immutable list.

Author: jyemin

driver-core/src/main/com/mongodb/ConnectionString.java

@@ -827,12 +827,12 @@ public String getConnectionString() {
 
 
     /**
-     * Gets the credentials.
+     * Gets the credentials in an immutable list.  The list will be empty if no credentials were specified in the connection string.
      *
-     * @return the credentials
+     * @return the credentials in an immutable list
      */
     public List<MongoCredential> getCredentialList() {
-        return credentials != null ? asList(credentials) : new ArrayList<MongoCredential>();
+        return credentials != null ? singletonList(credentials) : Collections.<MongoCredential>emptyList();
     }
 
     /**

driver-core/src/test/unit/com/mongodb/ConnectionStringSpecification.groovy

@@ -252,6 +252,18 @@ class ConnectionStringSpecification extends Specification {
                                                                           .withMechanismProperty('SERVICE_REALM', 'AWESOME'))
     }
 
+    @Unroll
+    def 'should create immutable credential list'() {
+        when:
+        uri.credentialList.add(createGSSAPICredential('user'))
+
+        then:
+        thrown(UnsupportedOperationException)
+
+        where:
+        uri << [new ConnectionString('mongodb://jeff:123@localhost'), new ConnectionString('mongodb://localhost')]
+    }
+
     def 'should support thrown an IllegalArgumentException when given invalid authMechanismProperties'() {
         when:
         new ConnectionString('mongodb://jeff@localhost/?' +