This commit fixes two problems:
1) When the "course/students" endpoint is called, in the response all the students are returned with a nulled out degreeId field.
2) The degree has the value that belongs to the degreeId field.

This happens because the degree value of this class (FenixCourseStudents) is being set two times.

 this.degree = degree.getSigla();
 this.degree = degree.getExternalId();

I fixed the two problems by changing the second this.degree to this.degreeId.