docs/resource/aws_lb: Add documentation for new drop_invalid_header flag

This commit adds the documentation for the new
drop_invalid_header_fields attribute for aws application load balancers.
Sourced from
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/application-load-balancers.html.

website/docs/r/lb.html.markdown

@@ -89,6 +89,7 @@ Terraform will autogenerate a name beginning with `tf-lb`.
 * `internal` - (Optional) If true, the LB will be internal.
 * `load_balancer_type` - (Optional) The type of load balancer to create. Possible values are `application` or `network`. The default value is `application`.
 * `security_groups` - (Optional) A list of security group IDs to assign to the LB. Only valid for Load Balancers of type `application`.
+* `drop_invalid_header_fields` - (Optional) Indicates whether HTTP headers with header fields that are not valid are removed by the load balancer (true) or routed to targets (false). The default is false. Elastic Load Balancing requires that message header names contain only alphanumeric characters and hyphens. Only valid for Load Balancers of type `application`.
 * `access_logs` - (Optional) An Access Logs block. Access Logs documented below.
 * `subnets` - (Optional) A list of subnet IDs to attach to the LB. Subnets
 cannot be updated for Load Balancers of type `network`. Changing this value