'Update the course\'s default minimum grade requirement for credit.
Args:
course_key(CourseKey): The course identifier
minimum_grade_json(Float): Minimum grade value
user(User): The user object'

@staticmethod
def update_minimum_grade_credit_from_json(course_key, minimum_grade_credit, user):

    descriptor = modulestore().get_course(course_key)
    if (minimum_grade_credit is not None):
        minimum_grade_credit = minimum_grade_credit
        descriptor.minimum_grade_credit = minimum_grade_credit
        modulestore().update_item(descriptor, user.id)
