Ensure the original hashes are not modified

The customer might pass a hash that is used someplace else in their
application. We should not modify the given hash.

(We just saw this issue with appsignal/appsignal-javascript#639)

lib/appsignal/logger.rb

@@ -162,7 +162,7 @@ def silence(_severity = ERROR, &block)
     attr_reader :default_attributes
 
     def add_with_attributes(severity, message, group, attributes)
-      Thread.current[:appsignal_logger_attributes] = default_attributes.merge!(attributes)
+      Thread.current[:appsignal_logger_attributes] = default_attributes.merge(attributes)
       add(severity, message, group)
     ensure
       Thread.current[:appsignal_logger_attributes] = nil

spec/lib/appsignal/logger_spec.rb

@@ -190,6 +190,17 @@
         Appsignal::Utils::Data.generate({ :other_key => "other_value", :some_key => "some_value" }))
       logger.error("Some message", { :other_key => "other_value" })
     end
+
+    it "does not modify the original attribute hashes passed" do
+      default_attributes = { :some_key => "some_value" }
+      logger = Appsignal::Logger.new("group", :attributes => default_attributes)
+
+      line_attributes = { :other_key => "other_value" }
+      logger.error("Some message", line_attributes)
+
+      expect(default_attributes).to eq({ :some_key => "some_value" })
+      expect(line_attributes).to eq({ :other_key => "other_value" })
+    end
   end
 
   describe "#error with exception object" do