Changed default content-type to text/html

spec/context_spec.cr

@@ -8,7 +8,7 @@ describe "Context" do
     end
     request = HTTP::Request.new("GET", "/")
     response = kemal.call(request)
-    response.headers["Content-Type"].should eq("text/plain")
+    response.headers["Content-Type"].should eq("text/html")
   end
 
   it "sets content type" do

src/kemal/context.cr

@@ -4,7 +4,7 @@ class Kemal::Context
   getter content_type
 
   def initialize(@request, @params)
-    @content_type = "text/plain"
+    @content_type = "text/html"
   end
 
   def set_content_type(content_type)