Throw error if getInitialProps is defined as as instance method

Omitting the static keyword happens pretty often. Therefore we should trigger a warning in devmode.

Closes: #4782

lib/utils.js

@@ -54,6 +54,14 @@ export function isResSent (res) {
 }
 
 export async function loadGetInitialProps (Component, ctx) {
+  if (process.env.NODE_ENV !== 'production') {
+    if (Component.prototype && Component.prototype.getInitialProps) {
+      const compName = getDisplayName(Component)
+      const message = `"${compName}.getInitialProps()" is defined as an instance method.`
+      throw new Error(message)
+    }
+  }
+
   if (!Component.getInitialProps) return {}
 
   const props = await Component.getInitialProps(ctx)