lib/subject/mapSubject.js
function KeyValue(subject, key) {
  this.subject = subject;
  this.key = key;
  this.format = subject.format;
  this.value = subject.value.get(key);
}
function KeyValue(subject, key) {
  this.subject = subject;
  this.key = key;
  this.format = subject.format;
  this.value = subject.value.get(key);
}
A fluent context object containing the value of the field that was just tested. Used for
additional assertions about a field.
@constructor