fix Environment#dump when construction variables are Symbols

lib/rscons/environment.rb

@@ -662,8 +662,10 @@ module Rscons
 
     # Print the Environment's construction variables for debugging.
     def dump
-      @varset.to_h.sort.each do |var, val|
-        puts "#{var} => #{val.inspect}"
+      varset_hash = @varset.to_h
+      varset_hash.keys.sort_by(&:to_s).each do |var|
+        var_str = var.is_a?(Symbol) ? var.inspect : var
+        puts "#{var_str} => #{varset_hash[var].inspect}"
       end
     end
 

spec/build_tests_spec.rb

@@ -725,9 +725,11 @@ EOF
     test_dir("simple")
     env = Rscons::Environment.new do |env|
       env["CFLAGS"] += %w[-O2 -fomit-frame-pointer]
+      env[:foo] = :bar
     end
     env.dump
     result = lines
+    expect(result.include?(%{:foo => :bar})).to be_truthy
     expect(result.include?(%{CFLAGS => ["-O2", "-fomit-frame-pointer"]})).to be_truthy
     expect(result.include?(%{CPPPATH => []})).to be_truthy
   end