Make ssl api more idiomatic

src/kemal/cli.cr

@@ -50,8 +50,8 @@ module Kemal
           exit
         end
         ssl = Kemal::SSL.new
-        ssl.set_key_file @key_file.not_nil!
-        ssl.set_cert_file @cert_file.not_nil!
+        ssl.key_file = @key_file.not_nil!
+        ssl.cert_file =  @cert_file.not_nil!
         Kemal.config.ssl = ssl.context
       end
     {% end %}

src/kemal/ssl.cr

@@ -6,11 +6,11 @@ module Kemal
       @context = OpenSSL::SSL::Context::Server.new
     end
 
-    def set_key_file(key_file)
+    def key_file=(key_file)
       @context.private_key = key_file
     end
 
-    def set_cert_file(cert_file)
+    def cert_file=(cert_file)
       @context.certificate_chain = cert_file
     end
   end