Fix derive with _ key

Summary:

Test Plan:

lib/encoder.ex

@@ -77,7 +77,7 @@ end
 defimpl Jason.Encoder, for: Any do
   defmacro __deriving__(module, struct, opts) do
     fields = fields_to_encode(struct, opts)
-    kv = Enum.map(fields, &{&1, generated_var(&1, __MODULE__)})
+    kv = Enum.map(fields, &{&1, generated_var(&1)})
     escape = quote(do: escape)
     encode_map = quote(do: encode_map)
     encode_args = [escape, encode_map]
@@ -94,9 +94,12 @@ defimpl Jason.Encoder, for: Any do
     end
   end
 
-  # The same as Macro.var/2 except it sets generated: true
-  defp generated_var(name, context) do
-    {name, [generated: true], context}
+  # The same as Macro.var/2 except it sets generated: true and handles _ key
+  defp generated_var(:_) do
+    {:__, [generated: true], __MODULE__.Underscore}
+  end
+  defp generated_var(name) do
+    {name, [generated: true], __MODULE__}
   end
 
   def encode(%_{} = struct, _opts) do

test/encode_test.exs

@@ -135,6 +135,11 @@ defmodule Jason.EncoderTest do
     defstruct name: "", size: 0
   end
 
+  defmodule DerivedWeirdKey do
+    @derive Encoder
+    defstruct [:_]
+  end
+
   defmodule NonDerived do
     defstruct name: ""
   end
@@ -154,6 +159,9 @@ defmodule Jason.EncoderTest do
 
     derived_using_except = %DerivedUsingExcept{name: "derived using :except", size: 10}
     assert to_json(derived_using_except) == ~s({"size":10})
+
+    derived_weird_key = %DerivedWeirdKey{}
+    assert to_json(derived_weird_key) == ~s({"_":null})
   end
 
   test "@derive validate `except:`" do