Fix immutable response header bug.

Node 18 responses have an immutable location header. Refactor logic to
avoid issue.

lib/documentLoaders/node.js

@@ -80,6 +80,7 @@ module.exports = ({
       url, headers, strictSSL, httpAgent, httpsAgent
     });
     doc = {contextUrl: null, documentUrl: url, document: body || null};
+
     // handle error
     const statusText = http.STATUS_CODES[res.status];
     if(res.status >= 400) {
@@ -92,7 +93,9 @@ module.exports = ({
         });
     }
     const link = res.headers.get('link');
+    let location = res.headers.get('location');
     const contentType = res.headers.get('content-type');
+
     // handle Link Header
     if(link && contentType !== 'application/ld+json') {
       // only 1 related link header permitted
@@ -115,10 +118,10 @@ module.exports = ({
         alternate.type == 'application/ld+json' &&
         !(contentType || '')
           .match(/^application\/(\w*\+)?json$/)) {
-        res.headers.set('location', prependBase(url, alternate.target));
+        location = prependBase(url, alternate.target);
       }
     }
-    const location = res.headers.get('location');
+
     // handle redirect
     if((alternate ||
       res.status >= 300 && res.status < 400) && location) {