avoid parsing the body if content-type is missing

[stevehu]

while working on issue #753, we have parsed the body even the content-type header is missing. This might not be the desired behaviour if the request is a get or delete which doesn't have a content-type. We cannot depend on users to config their endpoint correctly to only include body middleware handler to the PUT, POST and PATCH endpoints. So the decision is to only parse the body when content type exists in the header.

[stevehu]

avoid parsing the body if content-type is missing light-4j master

[stevehu]

avoid parsing the body if content-type is missing light-4j 1.6.x