Improve logging and specs

src/kemal/base_log_handler.cr

@@ -1,14 +1,7 @@
 module Kemal
   # All loggers must inherit from `Kemal::BaseLogHandler`.
-  class Kemal::BaseLogHandler < HTTP::Handler
-    def initialize
-    end
-
-    def call(context)
-      call_next context
-    end
-
-    def write(message)
-    end
+  abstract class Kemal::BaseLogHandler < HTTP::Handler
+    abstract def call(context)
+    abstract def write(message)
   end
 end

src/kemal/common_log_handler.cr

@@ -1,10 +1,9 @@
 module Kemal
   class CommonLogHandler < Kemal::BaseLogHandler
-    @handler : IO::FileDescriptor
-    getter handler
+    @handler : IO
 
-    def initialize
-      @handler = STDOUT
+    def initialize(io : IO)
+      @handler = io
     end
 
     def call(context)

src/kemal/config.cr

@@ -74,7 +74,7 @@ module Kemal
 
     private def setup_log_handler
       @logger ||= if @logging
-                    Kemal::CommonLogHandler.new
+                    Kemal::CommonLogHandler.new(STDOUT)
                   else
                     Kemal::NullLogHandler.new
                   end

src/kemal/null_log_handler.cr

@@ -1,5 +1,10 @@
 module Kemal
   # This is here to represent the logger corresponding to Null Object Pattern.
   class NullLogHandler < Kemal::BaseLogHandler
+    def call(context)
+    end
+
+    def write(message)
+    end
   end
 end