add test

apps/block_scout_web/lib/block_scout_web/chain.ex

@@ -33,6 +33,7 @@ defmodule BlockScoutWeb.Chain do
 
   defimpl Poison.Encoder, for: Decimal do
     def encode(value, _opts) do
+      # silence the xref warning
       decimal = Decimal
 
       [?\", decimal.to_string(value), ?\"]

apps/block_scout_web/test/block_scout_web/chain_test.exs

@@ -71,4 +71,12 @@ defmodule BlockScoutWeb.ChainTest do
       assert {:error, :not_found} = Chain.from_param(address_hash)
     end
   end
+
+  describe "Posion.encode!" do
+    test "correctly encodes decimal values" do
+      val = Decimal.from_float(5.55)
+
+      assert "\"5.55\"" == Poison.encode!(val)
+    end
+  end
 end