FIX: Allow Symbol objects to be deserialized in PostRevision (#17510)

Followup to ee07f6da7d111e46ca79454320d027c1aca7e157

spec/models/post_revision_spec.rb

@@ -23,4 +23,15 @@ describe PostRevision do
       }
     )
   end
+
+  it "can serialize and deserialize symbols" do
+    # Plugins may store symbolized values in this column
+    pr = Fabricate(:post_revision, modifications: { key: :value })
+    pr.reload
+    expect(pr.modifications).to eq(
+      {
+        key: :value
+      }
+    )
+  end
 end