Set content type to text/html for 500 exceptions (#616)

spec/exception_handler_spec.cr

@@ -59,6 +59,30 @@ describe "Kemal::ExceptionHandler" do
     response.body.should eq "Something happened"
   end
 
+  it "overrides the content type for filters" do
+    before_get do |env|
+      env.response.content_type = "application/json"
+    end
+    error 500 do |_, err|
+      err.message
+    end
+    get "/" do |env|
+      env.response.status_code = 500
+    end
+    request = HTTP::Request.new("GET", "/")
+    io = IO::Memory.new
+    response = HTTP::Server::Response.new(io)
+    context = HTTP::Server::Context.new(request, response)
+    Kemal::ExceptionHandler::INSTANCE.next = Kemal::RouteHandler::INSTANCE
+    Kemal::ExceptionHandler::INSTANCE.call(context)
+    response.close
+    io.rewind
+    response = HTTP::Client::Response.from_io(io, decompress: false)
+    response.status_code.should eq 500
+    response.headers["Content-Type"].should eq "text/html"
+    response.body.should eq "Rendered error with 500"
+  end
+
   it "keeps the specified error Content-Type" do
     error 500 do
       "Something happened"

src/kemal/helpers/templates.cr

@@ -22,6 +22,7 @@ def render_404
 end
 
 def render_500(context, exception, verbosity)
+  context.response.content_type = "text/html"
   context.response.status_code = 500
 
   template = if verbosity