Migrate all examples to use std::env::var_os (#312)

examples/print-request-response/src/main.rs

@@ -16,7 +16,7 @@ use tower::{filter::AsyncFilterLayer, util::AndThenLayer, BoxError};
 #[tokio::main]
 async fn main() {
     // Set the RUST_LOG, if it hasn't been explicitly defined
-    if std::env::var("RUST_LOG").is_err() {
+    if std::env::var_os("RUST_LOG").is_none() {
         std::env::set_var(
             "RUST_LOG",
             "example_print_request_response=debug,tower_http=debug",