Keep using kilt for rendering

Directly using Crystal's ECR seems to be causing issues, so
don't use kemal's 'render' macro and patch 'content_for' to
have the same behavior as before Kemal v1.1.1

src/ext/kemal_content_for.cr

@@ -0,0 +1,16 @@
+# Overrides for Kemal's `content_for` macro in order to keep using
+# kilt as it was before Kemal v1.1.1 (Kemal PR #618).
+
+require "kemal"
+require "kilt"
+
+macro content_for(key, file = __FILE__)
+  %proc = ->() {
+    __kilt_io__ = IO::Memory.new
+    {{ yield }}
+    __kilt_io__.to_s
+  }
+
+  CONTENT_FOR_BLOCKS[{{key}}] = Tuple.new {{file}}, %proc
+  nil
+end