Always set Content-Type to JSON

Fixes #45

lib/api.rb

@@ -65,6 +65,10 @@ options '*' do
   200
 end
 
+before do
+  content_type 'application/json'
+end
+
 get '/' do
   last_modified App.released_at
   jsonp details: 'http://fixer.io', version: App.version

spec/api_spec.rb

@@ -96,4 +96,11 @@ describe 'the API' do
     get '/latest'
     last_response.body.must_equal 'foo'
   end
+
+  it 'sets Content-Type header to JSON when caching' do
+    2.times do
+      get '/latest'
+      last_response.headers['Content-Type'].must_equal 'application/json'
+    end
+  end
 end