provider/aws: Lower case DB Instance's maintenance window

Fixes https://github.com/hashicorp/terraform/issues/2719

builtin/providers/aws/resource_aws_db_instance.go

@@ -139,6 +139,10 @@ func resourceAwsDbInstance() *schema.Resource {
 				Type:     schema.TypeString,
 				Optional: true,
 				Computed: true,
+				StateFunc: func(v interface{}) string {
+					value := v.(string)
+					return strings.ToLower(value)
+				},
 			},
 
 			"multi_az": &schema.Schema{

builtin/providers/aws/resource_aws_db_instance_test.go

@@ -183,6 +183,12 @@ resource "aws_db_instance" "bar" {
 	password = "barbarbarbar"
 	username = "foo"
 
+
+	# Maintenance Window is stored in lower case in the API, though not strictly 
+	# documented. Terraform will downcase this to match (as opposed to throw a 
+	# validation error).
+	maintenance_window = "Fri:09:00-Fri:09:30"
+
 	backup_retention_period = 0
 
 	parameter_group_name = "default.mysql5.6"