fix: ensure connection header is a string (#1900)

Author: ronag

lib/core/request.js

@@ -327,7 +327,7 @@ function processHeader (request, key, val) {
     key.length === 10 &&
     key.toLowerCase() === 'connection'
   ) {
-    const value = val.toLowerCase()
+    const value = typeof val === 'string' ? val.toLowerCase() : null
     if (value !== 'close' && value !== 'keep-alive') {
       throw new InvalidArgumentError('invalid connection header')
     } else if (value === 'close') {