Expect fill style to be handled (#4)

## Describe your changes
`fill` can be set via css alternatively to setting via the html fill
attribute.

## Checklist before requesting a review
- [x] I have performed a self-review of my code
- [x] If it is a core feature, I have added thorough tests.

Author: lounsbrough

src/ReactColorA11y.cy.tsx

@@ -63,7 +63,7 @@ describe('ReactColorA11y', () => {
         <ReactColorA11y flipBlackAndWhite>
           <svg height={100} width={100}>
             {expectedColorMappings.map(({ original }, index) => (
-              <circle id={index.toString()} key={original} cx={50} cy={50} r={50} stroke={original} fill={original} style={{ stroke: original }} />
+              <circle id={index.toString()} key={original} cx={50} cy={50} r={50} stroke={original} fill={original} style={{ stroke: original, fill: original }} />
             ))}
           </svg>
         </ReactColorA11y>
@@ -76,6 +76,7 @@ describe('ReactColorA11y', () => {
       cy.get(`#${index}`).then(($element) => {
         expectColorsToMatch($element.attr('fill'), lighter);
         expectColorsToMatch($element.attr('stroke'), lighter);
+        expectColorsToMatch($element.css('fill'), lighter);
         expectColorsToMatch($element.css('stroke'), lighter);
       });
     });

src/ReactColorA11y.tsx

@@ -153,10 +153,13 @@ const ReactColorA11y: React.FunctionComponent<ReactColorA11yProps> = ({
       element.setAttribute('stroke', calculateA11yColor(backgroundColor, strokeColor))
     }
 
-    const { color: computedColor, stroke: computedStroke } = getComputedStyle(element)
+    const { color: computedColor, stroke: computedStroke, fill: computedFill } = getComputedStyle(element)
     if (computedColor !== null) {
       element.style.color = calculateA11yColor(backgroundColor, computedColor)
     }
+    if (computedFill !== null) {
+      element.style.fill = calculateA11yColor(backgroundColor, computedFill)
+    }
     if (computedStroke !== null) {
       element.style.stroke = calculateA11yColor(backgroundColor, computedStroke)
     }