fix: support non-lower-case Content-Type header provided by app (#516)

* fix: support non-lower-case Content-Type header provided by app

The changes in #511 change what headers get set by PDFKit
and make all headers set by PDFKit lower-case.

However, the changes also affect code depending on headers
set by the app - which for Rack 2.x apps will be 'Content-Type'.

To address this, the code should check if Content-Type is present
and in that case use it, otherwise default to content-type.

As the code also sets this header (changing the content type
to 'application/pdf'), it should set the same header that
the original value is retrieved from.

So decide on the exact header name first, store it in the
'content_type_header' variable, and use it to index the
headers dict.

Fix #515.

* rspec: add tests for mixed case Content-Type header support

As rack 3.x outright rejects mixed case headers, this test has to be marked
as pending - but passes (without the pending flag) with rack 2.x.

* rspec/content-type: make mixed case test pending only on Rack>=3.0.0

lib/pdfkit/middleware.rb

@@ -22,7 +22,8 @@ def _call(env)
       status, headers, response = @app.call(env)
 
       begin
-        if rendering_pdf? && headers['content-type'] =~ /text\/html|application\/xhtml\+xml/
+        content_type_header = headers.has_key?('Content-Type') ? 'Content-Type' : 'content-type'
+        if rendering_pdf? && headers[content_type_header] =~ /text\/html|application\/xhtml\+xml/
           body = response.respond_to?(:body) ? response.body : response.join
           body = body.join if body.is_a?(Array)
 
@@ -49,7 +50,7 @@ def _call(env)
           end
 
           headers['content-length'] = (body.respond_to?(:bytesize) ? body.bytesize : body.size).to_s
-          headers['content-type']   = 'application/pdf'
+          headers[content_type_header]   = 'application/pdf'
           headers['content-disposition'] ||= @conditions[:disposition] || 'inline'
         end
       rescue StandardError => e

spec/middleware_spec.rb

@@ -422,6 +422,29 @@ def mock_app(options = {}, conditions = {}, custom_headers = {})
       end
     end
 
+    describe "content type header" do
+      before { mock_app }
+
+      context "lower case" do
+        specify "header gets correctly updated" do
+          get 'http://www.example.org/public/test.pdf'
+          expect(last_response.headers["content-type"]).to eq("application/pdf")
+        end
+      end
+
+      context "mixed case" do
+        let(:headers) do
+          {'Content-Type' => "text/html"}
+        end
+
+        specify "header gets correctly updated" do
+          pending("this test only applies to rack 2.x and is rejected by rack 3.x") if Rack.release >= "3.0.0"
+          get 'http://www.example.org/public/test.pdf'
+          expect(last_response.headers["Content-Type"]).to eq("application/pdf")
+        end
+      end
+    end
+
     describe "remove .pdf from PATH_INFO and REQUEST_URI" do
       before { mock_app }