[routing-utils] Ensure headers with only has items are replaced correctly (#6686)

This ensures we replace header values correctly when no named segments are used and only has items are used. 

### Related Issues

Fixes: https://vercel.slack.com/archives/CHTTGQYQ4/p1631023974185700

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packages/routing-utils/src/superstatic.ts

@@ -112,7 +112,7 @@ export function convertHeaders(headers: Header[]): Route[] {
     });
 
     h.headers.forEach(({ key, value }) => {
-      if (namedSegments.length > 0) {
+      if (namedSegments.length > 0 || hasSegments.length > 0) {
         if (key.includes(':')) {
           key = safelyCompile(key, indexes);
         }