fix Crystal 1.5.0 warnings

spec/spec_helper.cr

@@ -4,8 +4,8 @@ require "../src/*"
 include Kemal
 
 class CustomLogHandler < Kemal::BaseLogHandler
-  def call(env)
-    call_next env
+  def call(context)
+    call_next(context)
   end
 
   def write(message)

src/kemal/handler.cr

@@ -27,8 +27,8 @@ module Kemal
       end
     end
 
-    def call(env : HTTP::Server::Context)
-      call_next(env)
+    def call(context : HTTP::Server::Context)
+      call_next(context)
     end
 
     # Processes the path based on `only` paths which is a `Array(String)`.