fix(lineargradient): fix issue with rgba value as the first color stop

Fixes issue with rgba values having spaces and being used at the first color stop value.

fix #538

package.json

@@ -1,6 +1,6 @@
 {
   "name": "polished",
-  "version": "3.6.6",
+  "version": "3.6.7",
   "description": "A lightweight toolset for writing styles in Javascript.",
   "license": "MIT",
   "author": "Brian Hough <hello@brianhough.net> (https://polished.js.org)",

src/mixins/linearGradient.js

@@ -43,9 +43,14 @@ export default function linearGradient({
     throw new PolishedError(56)
   }
   return {
-    backgroundColor: fallback || colorStops[0].split(' ')[0],
-    backgroundImage: constructGradientValue`linear-gradient(${toDirection}${colorStops.join(
-      ', ',
-    )})`,
+    backgroundColor:
+      fallback
+      || colorStops[0]
+        .replace(/,\s+/g, ',')
+        .split(' ')[0]
+        .replace(/,(?=\S)/g, ', '),
+    backgroundImage: constructGradientValue`linear-gradient(${toDirection}${colorStops
+      .join(', ')
+      .replace(/,(?=\S)/g, ', ')})`,
   }
 }