Follow the standard to handle charset

[alalag1]

Motivation

In Restlight, it seems that StandardCharsets.UTF_8 is the default charset, but we should flow the HTTP standard to handle the charset such as content-type header, accept header.

And Here are some wrong uses

• How can we make sure that UTF-8 is compatible with the given value
  

  esa-restlight/restlight-core/src/main/java/esa/restlight/core/serialize/GsonSerializer.java

  Lines 43 to 45 in ce8fe48

  	public byte[] serialize(Object target) {
  	return gson.toJson(target).getBytes(StandardCharsets.UTF_8);
  	}

• We should try to retrieve charset from the content-type rather than using UTF8 directly
  

  esa-restlight/restlight-core/src/main/java/esa/restlight/core/resolver/arg/AbstractRequestBodyArgumentResolver.java

  Lines 98 to 107 in ce8fe48

  	MediaType contentType = getMediaType(request);
  	//convert argument if content-type is text/plain or missing.
  	if (contentType == null || MediaType.TEXT_PLAIN.isCompatibleWith(contentType)) {
  	//ignore empty body.
  	if (request.byteBufBody().readableBytes() == 0) {
  	return checkRequired(null);
  	}
  	return checkRequired(converter.apply(request.byteBufBody()
  	.toString(StandardCharsets.UTF_8)));
  	}

We should re-examine the handling of charset in this whole project