Fixed Array.prototype.concat() when "this" is a slow array.

Previously, when the current appended element is fast array the "this"
array was expected to always be a fast array also.  This may not be the
case when the previous appended element was not fast thus converting
the "this" array to a slow form.

Previous fix introduced in 2c1382bab643 (0.7.2) was not complete, the
correct fix is to never assume "this" is fast, whereas njs_array_add()
may only be called with fast arrays.

This closes #471 issue in Github.