fix(ssr): should not render invalid numeric style values

Reverts part of 7d9cfebe - browsers only auto append units to
numbers when in quirksmode, so in standard mode, numbers set to
style properties that require units are invalid and should not
be rendered.

src/platforms/web/server/modules/style.js

@@ -21,18 +21,11 @@ export function genStyle (style: Object): string {
 }
 
 function normalizeValue(key: string, value: any): string {
-  if (typeof value === 'string') {
+  if (
+    typeof value === 'string' ||
+    (typeof value === 'number' && noUnitNumericStyleProps[key])
+  ) {
     return `${key}:${value};`
-  } else if (typeof value === 'number') {
-    // Handle numeric values.
-    // Turns out all evergreen browsers plus IE11 already support setting plain
-    // numbers on the style object and will automatically convert it to px if
-    // applicable, so we should support that on the server too.
-    if (noUnitNumericStyleProps[key]) {
-      return `${key}:${value};`
-    } else {
-      return `${key}:${value}px;`
-    }
   } else {
     // invalid values
     return ``

test/ssr/ssr-string.spec.js

@@ -1528,14 +1528,14 @@ describe('SSR: renderToString', () => {
       template: '<div :style="style"></div>',
       data: {
         style: {
-          opacity: 0, // opacity should display as-is
-          top: 0, // top and margin should be converted to '0px'
-          marginTop: 10
+          opacity: 0, // valid, opacity is unit-less
+          top: 0, // invalid, top requires unit
+          marginTop: '10px' // valid
         }
       }
     }, result => {
       expect(result).toContain(
-        '<div data-server-rendered="true" style="opacity:0;top:0px;margin-top:10px;"></div>'
+        '<div data-server-rendered="true" style="opacity:0;margin-top:10px;"></div>'
       )
       done()
     })