Stop adding extra attributes to user defined HttpException

[feroult]

Use as the response body the user defined text/object (json serialized) without any modifications.

Think about add an option for mime-type.

[feroult]

Examples:

throw new HttpException(500, validationObject); throw new HttpException(500, "something wrong", "text/plain"); throw new HttpException(500, "{ 'attr': 'value' }"); // default is text/json

[feroult]

Fixed yawp-1.3.15

Works only with strings and text/json, same as worked before.