fix encoding issue on windows+jruby

Fixes #2507

lib/logstash/environment.rb

@@ -117,6 +117,10 @@ module LogStash
       @jruby ||= !!(RUBY_PLATFORM == "java")
     end
 
+    def windows?
+      Gem.win_platform?
+    end
+
     def vendor_path(path)
       return ::File.join(LOGSTASH_HOME, "vendor", path)
     end

lib/logstash/event.rb

@@ -296,6 +296,7 @@ class LogStash::Event
   def self.validate_value(value)
     case value
     when String
+      value.force_encoding("UTF-8") if LogStash::Environment.windows?
       raise("expected UTF-8 encoding for value=#{value}, encoding=#{value.encoding.inspect}") unless value.encoding == Encoding::UTF_8
       raise("invalid UTF-8 encoding for value=#{value}, encoding=#{value.encoding.inspect}") unless value.valid_encoding?
       value