code: function(http, extensions, defaultOptions) {
    this.http = http;
    this.extensions = extensions;
    this.defaultOptions = defaultOptions || {};
    this.headers = {};
  }
docs: Create a new context with the given http abstraction and set of
  extensions
  @constructor
  
  @classdesc
  The {@link Context} encapsulates the state/context of a given resource
  or request, including extensions available, the http mechanism for
  dereferencing URLs, etc.