Inherit response.content type from filter if it's given. Fixes #146.

spec/route_handler_spec.cr

@@ -155,4 +155,13 @@ describe "Kemal::RouteHandler" do
     client_response.status_code.should eq(302)
     client_response.headers.has_key?("Location").should eq(true)
   end
+
+  it "sets default Content-Type to context html" do
+    get "/" do |env|
+      "Hello World from GET"
+    end
+    request = HTTP::Request.new("GET", "/")
+    client_response = call_request_on_app(request)
+    client_response.content_type.should eq("text/html")
+  end
 end

src/kemal/route_handler.cr

@@ -13,7 +13,7 @@ class Kemal::RouteHandler < HTTP::Handler
   end
 
   def call(context)
-    context.response.content_type = "text/html"
+    context.response.content_type = "text/html" unless context.response.headers.has_key?("Content-Type")
     process_request(context)
   end