Log unknown severity levels at info level

lenjador#add accepts `severity`. Fall back to `info` when the severity
cannot be mapped to a known "level" to not lose messages.

This was motivated by an external tool calling #add with 'any' severity.

lib/lenjador.rb

@@ -29,7 +29,7 @@ def initialize(adapter, level, preprocessors)
   end
 
   def add(severity, *args, &block)
-    level = SEV_LABEL.index(severity.to_s)
+    level = SEV_LABEL.index(severity.to_s) || SEV_LABEL.index('info')
     log(level, *args, &block)
   end
 

spec/lenjador_spec.rb

@@ -49,6 +49,11 @@
       lenjador.add('info', 'info-msg')
       lenjador.add('warn', 'warn-msg')
     end
+
+    it 'logs messages with unknown severity at info level' do
+      expect(adapter).to receive(:log).with(described_class::Severity::INFO, message: 'unknown-msg').ordered
+      lenjador.add('something-else', 'unknown-msg')
+    end
   end
 
   context 'when preprocessor defined' do