[ruby/open-uri] Update error message for `request_specific_fields` option validation

Added `inspect` to the `request_specific_fields` value to provide
better visibility for users in the exception message.

https://github.com/ruby/open-uri/commit/f89ce5112d

lib/open-uri.rb

@@ -151,7 +151,7 @@ module OpenURI
     end
     if options.has_key? :request_specific_fields
       if !(options[:request_specific_fields].is_a?(Hash) || options[:request_specific_fields].is_a?(Proc))
-        raise ArgumentError, "Invalid request_specific_fields' format: #{options[:request_specific_fields]}"
+        raise ArgumentError, "Invalid request_specific_fields option: #{options[:request_specific_fields].inspect}"
       end
     end
     unless mode == nil ||

test/open-uri/test_open-uri.rb

@@ -393,7 +393,7 @@ class TestOpenURI < Test::Unit::TestCase
     with_http {|srv, url|
       srv.mount_proc("/r1/", lambda {|req, res| res.body = "r1" } )
       exc = assert_raise(ArgumentError) { URI.open("#{url}/r1/", request_specific_fields: "dummy_token") {} }
-      assert_equal("Invalid request_specific_fields' format: dummy_token", exc.message)
+      assert_equal('Invalid request_specific_fields option: "dummy_token"', exc.message)
     }
   end