Allow empty attributes

README.md

@@ -271,15 +271,15 @@ iex> TwiML.say("Sorry, calls are currently unavailable")
 """
 ```
 
-Empty attributes are not included in the generated TwiML:
+Attributes with a value of nil are excluded from the generated TwiML:
 
 ```elixir
-iex> TwiML.say("Hello", voice: "", loop: nil)
+iex> TwiML.gather(input: "dtmf", finish_on_key: "", num_digits: nil)
 ...> |> TwiML.to_xml()
 """
 <?xml version="1.0" encoding="UTF-8"?>
 <Response>
-  <Say>Hello</Say>
+  <Gather input="dtmf" finishOnKey=""/>
 </Response>\
 """
 ```

lib/twiml.ex

@@ -79,15 +79,14 @@ defmodule TwiML do
   end
 
   defp build_verb(verb, attrs, children) do
-    verb = verb |> Atom.to_string() |> String.capitalize()
+    verb =
+      verb
+      |> Atom.to_string()
+      |> String.capitalize()
 
     attrs =
       attrs
-      |> Enum.reject(fn
-        {_, nil} -> true
-        {_, ""} -> true
-        _ -> false
-      end)
+      |> Enum.reject(&is_nil(elem(&1, 1)))
       |> Enum.map(fn {k, v} -> {camelize(k, :lower), v} end)
 
     {verb, attrs, children}