mfenced element is not equivalent to its expanded form

Created by: fred-wang

Per the MathML spec http://www.w3.org/TR/MathML3/chapter3.html#presm.mfenced

<math>
  <mfenced open="&#x2225;" close="&#x2223;">
    <mfrac>
      <mi>x</mi>
      <mi>y</mi>
    </mfrac>
  </mfenced>
</math>

should be equivalent to

<math>
  <mrow>
   <mo fence="true">&#x2225;</mo>
   <mrow>
     <mfrac>
       <mi>x</mi>
       <mi>y</mi>
     </mfrac>
   </mrow>
   <mo fence="true">&#x2223;</mo>
  </mrow>
</math>

but that's not the case (spacing, strechiness). This is the cause of regression #475 (closed).

(in general inconsistency between mfenced and the mrow equivalent construction in all MathML implementations is one reason that motivates deprecating mfenced and working only on mrow+mo)