This fixes warnings on Ruby 2.7 (#1009)

lib/faraday/adapter/em_http.rb

@@ -229,11 +229,11 @@ def running?
           @running
         end
 
-        def add
+        def add(&block)
           if running?
             perform_request { yield }
           else
-            @registered_procs << Proc.new
+            @registered_procs << block
           end
           @num_registered += 1
         end

lib/faraday/options.rb

@@ -79,7 +79,7 @@ def fetch(key, *args)
         if !args.empty?
           send(key_setter, args.first)
         elsif block_given?
-          send(key_setter, Proc.new.call(key))
+          send(key_setter, yield(key))
         else
           raise self.class.fetch_error_class, "key not found: #{key.inspect}"
         end

lib/faraday/response.rb

@@ -55,9 +55,9 @@ def finished?
       !!env
     end
 
-    def on_complete
+    def on_complete(&block)
       if !finished?
-        @on_complete_callbacks << Proc.new
+        @on_complete_callbacks << block
       else
         yield(env)
       end