Fix faulty admin spinner animation

When the @spinner-delay variable does not have a unit (s/seconds), the generated animation-delay attribute for the western spinner spoke also lacks a unit. In certain browsers (I am testing with Firefox Developer Edition 59.0b10 (64-bit)), this CSS style is marked as invalid and not applied, causing the western and south-western spokes of the spinner to flash out of order in the resulting animation. I assume that, somewhere in the LESS calculations, the @spinner-delay and @spinner-animation-step variables are combined with arithmetic, coercing the resulting spoke style to have the correct units: this coercion does not occur for the final spoke, perhaps because the arithmetic involved does not include the last child (for which the animation delay should be at a maximum).

In any case, the error is hardly noticeable, but there's no reason to let such a easily-fixable styling omission slip through the cracks.

Author: neillrobson

app/design/adminhtml/Magento/backend/web/css/source/variables/_spinner.less

@@ -26,4 +26,4 @@
 @spinner-rotate-step: 45;
 
 //  One step in degree
-@spinner-delay: .9;
+@spinner-delay: .9s;