subscriber: use INFO as the default log level in EnvFilter

The previous implementation would fall back to "NOTHING" instead, which
was a poor default.

Close #735

Author: ishitatsuyuki

tracing-subscriber/src/filter/env/mod.rs

@@ -250,7 +250,9 @@ impl EnvFilter {
         use tracing::level_filters::STATIC_MAX_LEVEL;
         use tracing::Level;
 
-        let directives: Vec<_> = directives.into_iter().collect();
+        let fallback_level = Directive::from(crate::fmt::Subscriber::DEFAULT_MAX_LEVEL);
+
+        let directives: Vec<_> = std::iter::once(fallback_level).chain(directives.into_iter()).collect();
 
         let disabled: Vec<_> = directives
             .iter()