Get correct expression string when Zend\Db\Sql\Expression is used

[Martin-P]

Fix for #7407.

The processing of Zend\Db\Sql\Expression assumed the value of $part was the correct value to be used, however when an instance of Zend\Db\Sql\Expression is provided the correct value is in Zend\Db\Sql\Expression::getExpression().

[freax]

May be it'll be better:

if (is_string($part)) {
    if ($expression instanceof Expression) {
        // #7407: use $expression->getExpression() to get the unescaped version of the expression
        $sql .= $expression->getExpression();
    } else {
        // simply tack it onto the return sql "specification" string
        $sql .= $part;
    }
} else (is_array($part)) {
    //
} else {
    throw // Elements returned from getExpressionData() array must be a string or array.
}

[Martin-P]

Also possible, but I tried to avoid nesting if.

[freax]

To me It looks better then continue and we get rid of throw in a middle of a function

[Martin-P]

When looking at different pull requests I often see the preferred way is to use continue instead of if/elseif/else structures. Personally I find the continue easier to read, but it is all just a matter of preference.

For now I will leave it as it is, but if the ZF2 maintainers want the other structure it is no problem and I can change it at that moment.