multivar polynomials over poly fraction fields don't simplify

<div id="comment:0"></div>


```
sage: F=FractionField(PolynomialRing(QQ,'t'))
sage: R=PolynomialRing(F,'x,y')
sage: R.inject_variables() 
Defining x, y
sage: x/x 
x/x
```
whereas simpler fraction fields for F yield 1

CC:  @sagetrac-jakobkroeker

Component: **commutative algebra**

_Issue created by migration from https://trac.sagemath.org/ticket/15971_

