bug in new ring extension constructor #1040

Now that we allow notation such as

QQ[2^(1/3)]

to create a number field, the following is totally wrong and
needs to be fixed ASAP:

sage: K.<a> = QQ[2^(1/3)]
sage: K
Univariate Polynomial Ring in a over Rational Field
sage: parent(a)
Univariate Polynomial Ring in a over Rational Field

Component: number theory

Issue created by migration from https://trac.sagemath.org/ticket/1040