Preserve empty JSON types

The existing behavior when specifying `serializer: :json` was to store
and return an empty Hash (`{}`) for null or empty values, including
empty arrays.

This changes the encode logic to store the JSON representation of
any given value. During decode, any valid JSON string can be decoded. We
special case `nil` or `""` (empty string) to return `nil` (instead of
`{}` previously).

This could be a breaking for applications that depend on the empty hash
== nil behavior.

lib/vault/rails/json_serializer.rb

@@ -9,15 +9,14 @@ module JSONSerializer
       def self.encode(raw)
         self._init!
 
-        raw = {} if raw.nil?
-
         JSON.fast_generate(raw)
       end
 
       def self.decode(raw)
         self._init!
 
-        return {} if raw.nil? || raw.empty?
+        return nil if raw == nil || raw == ""
+
         JSON.parse(raw, DECODE_OPTIONS)
       end
 

spec/lib/vault/rails/json_serializer_spec.rb

@@ -0,0 +1,42 @@
+require 'spec_helper'
+
+RSpec.describe Vault::Rails::JSONSerializer do
+  [
+    nil,
+    false,
+    true,
+    "",
+    "foo",
+    {},
+    { "foo" => "bar" },
+    [],
+    ["foo", "bar"],
+    0,
+    123,
+    0.0,
+    0.123,
+    0xff,
+    123e123
+  ].each do |object|
+    it "encodes and decodes #{object.inspect}" do
+      encoded = described_class.encode(object)
+      expect(encoded).to be_a(String)
+      decoded = described_class.decode(encoded)
+      expect(decoded).to eq(object)
+    end
+  end
+
+  describe ".decode" do
+    subject(:decoded) { described_class.decode(raw) }
+
+    context "with nil" do
+      let(:raw) { nil }
+      it { is_expected.to eq(nil) }
+    end
+
+    context "with an empty string" do
+      let(:raw) { "" }
+      it { is_expected.to eq(nil) }
+    end
+  end
+end