“ It is contended, however, that the complainant is not the assignee of the contract of May 31, 1871, but a mortgagee in trust of the lands mentioned therein, and can therefore maintain the suit by reason of his citizenship in New York. We cannot assent to this position. It is true the complainant is a mortgagee in trust of such interest as the mortgagor had in the lands, but he brings the suit, not to foreclose the mortgage, but as one having a beneficial interest in the contract, a d consequently a right to enforce it. ”