Extract the SEVERITY_NAMES constant

This array lists the lowercase names of the supported severities which
can be set on the Logger.

lib/rackstash.rb

@@ -17,6 +17,15 @@ module Rackstash
     UNKNOWN = 5
   ].freeze
 
+  SEVERITY_NAMES = {
+    'debug' => DEBUG,
+    'info' => INFO,
+    'warn' => WARN,
+    'error' => ERROR,
+    'fatal' => FATAL,
+    'unknown' => UNKNOWN
+  }.freeze
+
   SEVERITY_LABELS = [
     'DEBUG'.freeze,
     'INFO'.freeze,

lib/rackstash/logger.rb

@@ -82,20 +82,7 @@ module Rackstash
       if severity.is_a?(Integer)
         @level = severity
       else
-        case severity.to_s.downcase
-        when 'debug'.freeze
-          @level = DEBUG
-        when 'info'.freeze
-          @level = INFO
-        when 'warn'.freeze
-          @level = WARN
-        when 'error'.freeze
-          @level = ERROR
-        when 'fatal'.freeze
-          @level = FATAL
-        when 'unknown'.freeze
-          @level = UNKNOWN
-        else
+        @level = SEVERITY_NAMES.fetch(severity.to_s.downcase) do
           raise ArgumentError, "invalid log level: #{severity}"
         end
       end