Always set the axis innermost property.

It doesn't make sense to calculate the innermost property only when no
tickLength has been provided; the two have little to do with each other.
Fix incorrect check for the innermost axis.

An axis is innermost when it's first among those not just in its
direction, but also on its side, i.e. left/right, of the plot. So the
inArray check should be against the samePosition list, not all.
Resolves #1056.
Updated credits for the axis innermost fix.