Stringify configuration overrides before first use

This makes sure that overrides for Jekyll.configuration
all have string keys before their first use, particularly
also the "config" and "skip_config_files" options.

lib/jekyll.rb

@@ -101,6 +101,7 @@ module Jekyll
     # Returns the final configuration Hash.
     def configuration(override = {})
       config = Configuration.new
+      override = Configuration[override].stringify_keys
       unless override.delete("skip_config_files")
         config = config.read_config_files(config.config_files(override))
       end

test/test_configuration.rb

@@ -334,6 +334,18 @@ class TestConfiguration < JekyllUnitTest
         Jekyll.configuration(test_config.merge({ "config" => @paths[:other] }))
     end
 
+    should "load different config if specified with symbol key" do
+      allow(SafeYAML).to receive(:load_file).with(@paths[:default]).and_return({})
+      allow(SafeYAML)
+        .to receive(:load_file)
+        .with(@paths[:other])
+        .and_return({ "baseurl" => "http://example.com" })
+      allow($stdout).to receive(:puts).with("Configuration file: #{@paths[:other]}")
+      assert_equal \
+        site_configuration({ "baseurl" => "http://example.com" }),
+        Jekyll.configuration(test_config.merge({ :config => @paths[:other] }))
+    end
+
     should "load default config if path passed is empty" do
       allow(SafeYAML).to receive(:load_file).with(@paths[:default]).and_return({})
       allow($stdout).to receive(:puts).with("Configuration file: #{@paths[:default]}")