Make sure that the function that checks if the content-type header

is valid for parsing the response body
and supports when there are no content-type header information.

lib/datadog/appsec/contrib/rack/gateway/response.rb

@@ -66,7 +66,7 @@ def response
 
             def json_content_type?
               content_type = headers['content-type']
-              VALID_JSON_TYPES.any? { |valid_type| content_type.include?(valid_type) }
+              VALID_JSON_TYPES.include?(content_type)
             end
           end
         end

spec/datadog/appsec/contrib/rack/gateway/response_spec.rb

@@ -8,12 +8,13 @@
 RSpec.describe Datadog::AppSec::Contrib::Rack::Gateway::Response do
   let(:body) { ['Ok'] }
   let(:content_type) { 'text/html' }
+  let(:headers) { { 'Content-Type' => content_type } }
 
   let(:response) do
     described_class.new(
       body,
       200,
-      { 'Content-Type' => content_type },
+      headers,
       scope: instance_double(Datadog::AppSec::Scope)
     )
   end
@@ -93,6 +94,14 @@
         end
       end
 
+      context 'without content-type header' do
+        let(:headers) { {} }
+
+        it 'returns nil' do
+          expect(response.parsed_body).to be_nil
+        end
+      end
+
       context 'with a body that is not an Array' do
         let(:body) { proc { ' "ba' } }