One could allow 3 conventions 
 1) a parallelogram is never a trapezoid 
 2) rectangles are trapezoids but not other paralleograms are 
 3) all parallelograms are trapezoids 
 I am fairly sure that almost every current US textbook uses convention 1). So that should maybe be noted to avoid confusing students (hence I would revert the latest change). I personally think 2) makes sense since the main theorem becomes (parallel bases + equal base angles)<=>(parallel bases + equal legs)<=>isosceles trapezoid. Also then every quadrilateral in the trapezoid rule is a trapezoid! Since b-a=0 in this case, some of the formulas (with (b-a)^2 in a denominator) need a fix (cancel out the (b-a)^2 from the numerator). (gentlemath at a public computer)