implemented API create endpoint

+ added plugins to read json and access the header to parse the request
  for an API request
+ fixed logic where url param is nil
+ refactored the new code to a variable to be reused easier
+ if CONTENT_TYPE header is application/json, reply with json
- there could be more refactoring (maybe separating it to a different
  endpoint) to handle other scenarios, we'll work on that later

app.rb

@@ -7,6 +7,8 @@ class App < Roda
   plugin :sessions, secret: ENV.delete('APP_SESSION_SECRET')
   plugin :render, escape: true
   plugin :flash
+  plugin :json_parser
+  plugin :request_headers
 
   DB = Sequel.sqlite("db/#{ENV['DB_NAME']}")
   links = DB[:links]
@@ -27,7 +29,7 @@ class App < Roda
 
     r.post "create" do
       url = r.params['url']
-      if url.empty? 
+      if url.nil? or url.empty? 
         flash['message'] = "Please enter a valid URL";
         r.redirect '/'
       end
@@ -36,8 +38,12 @@ class App < Roda
         links.insert(url: url, code: code) 
         @message = "Link created"
       end
+      code = links.filter(:url => url).first[:code]
       @message ||= "Link exists"
-      @new_link = 'http://' + request.env['HTTP_HOST'] + '/' + links.filter(:url => url).first[:code]
+      @new_link = 'http://' + request.env['HTTP_HOST'] + '/' + code
+      if 'application/json' == r.headers['CONTENT_TYPE']
+        return {url: url, code: code, link: @new_link}.to_json
+      end
       view :create   
     end
   end