Fix mix with string keys ending in ! (#967)

Closes #963

Author: joeldrapper

lib/phlex/helpers.rb

@@ -36,7 +36,14 @@ module Phlex::Helpers
 			end
 
 			result.transform_keys! do |key|
-				key.end_with?("!") ? key.name.chop.to_sym : key
+				case key
+				when Symbol
+					key.end_with?("!") ? key.name.chop.to_sym : key
+				when String
+					key.end_with?("!") ? key.chop.to_sym : key
+				else
+					key
+				end
 			end
 		end
 	end

quickdraw/helpers/mix.test.rb

@@ -65,11 +65,16 @@
 	assert_equal output, { class: ["foo", "bar"] }
 end
 
-test "override" do
+test "override with symbol key" do
 	output = mix({ class: "foo" }, { class!: "bar" })
 	assert_equal output, { class: "bar" }
 end
 
+test "override with string key" do
+	output = mix({ class: "foo" }, { "class!" => "bar" })
+	assert_equal output, { class: "bar" }
+end
+
 test "set + set" do
 	output = mix({ class: Set["foo"] }, { class: Set["bar"] })
 	assert_equal output, { class: Set["foo", "bar"] }