When constructing a Context from a HeaderMap, fill in the env_config field.

Author: khuey

lambda/src/types.rs

@@ -136,6 +136,7 @@ impl TryFrom<HeaderMap> for Context {
                 .to_str()
                 .expect("Invalid XRayTraceID sent by Lambda; this is a bug")
                 .to_owned(),
+            env_config: Config::from_env()?,
             ..Default::default()
         };
         Ok(ctx)