add ability for redirects to preserve the querystring and hash

when the requested URL is redirected, as the result of:
  config.redirects
preserve the querystring and hash, conditional to:
  config.redirects[index].preserveQuery
  config.redirects[index].preserveHash

when the requested URL is redirected, as the result of either:
  config.cleanUrls
  config.trailingSlash
preserve the querystring and hash, without condition

based on:
* upstream PR 154
    vercel/serve-handler#154
* upstream PR 144
    vercel/serve-handler#144

lib/serve-handler/src/index.js

@@ -139,7 +139,9 @@ const shouldRedirect = (decodedPath, {redirects = [], trailingSlash}, cleanUrl)
     }
     return {
       target: ensureSlashStart(decodedPath),
-      statusCode: defaultType
+      statusCode: defaultType,
+      preserveQuery: true,
+      preserveHash:  true
     }
   }
 
@@ -164,21 +166,25 @@ const shouldRedirect = (decodedPath, {redirects = [], trailingSlash}, cleanUrl)
     if (target) {
       return {
         target: ensureSlashStart(target),
-        statusCode: defaultType
+        statusCode: defaultType,
+        preserveQuery: true,
+        preserveHash:  true
       }
     }
   }
 
   // This is currently the fastest way to
   // iterate over an array
   for (let index = 0; index < redirects.length; index++) {
-    const {source, destination, type} = redirects[index]
+    const {source, destination, type, preserveQuery, preserveHash} = redirects[index]
     const target = toTarget(source, destination, decodedPath)
 
     if (target) {
       return {
         target,
-        statusCode: type || defaultType
+        statusCode:    type || defaultType,
+        preserveQuery: !!preserveQuery,
+        preserveHash:  !!preserveHash
       }
     }
   }
@@ -763,8 +769,18 @@ module.exports = async (request, response, config = {}, methods = {}) => {
   }
 
   if (paths.redirect) {
+    // conditionally preserve search and hash
+    let req_url, req_search, req_hash
+
+    if (paths.redirect.preserveQuery || paths.redirect.preserveHash) {
+      req_url = url.parse(request.url)
+    }
+
+    req_search = (paths.redirect.preserveQuery && req_url.search) ? req_url.search : ''
+    req_hash   = (paths.redirect.preserveHash  && req_url.hash)   ? req_url.hash   : ''
+
     response.writeHead(paths.redirect.statusCode, {
-      Location: encodeURI(paths.redirect.target)
+      Location: encodeURI(paths.redirect.target + req_search + req_hash)
     })
 
     response.end()