Only allow client to force unconditional request if options permit it

Koa's `ctx.fresh` uses https://github.com/jshttp/fresh/blob/v0.5.2/index.js#L43-L49
which always honors `Cache-Control: no-cache` in the request header.

That's unfortunate, because Chrome passes a `Cache-Control: no-cache` request header if you check "Disable cache" in Chrome Dev Tools, making it difficult/confusing to verify that nginx conditional requests are working.

In this commit, we use `ctx.fresh` only if our `options` object allows nginx itself to support bypassing the cache.

If we don't support clients bypassing the cache, then we'll return `304 Not Modified` if `if-modified-since` matches `last-modified`, regardless of what other headers the client sends.

app/src/app.js

@@ -91,7 +91,7 @@ export default class UnboxApp {
 
         this.app.use(async (ctx, next) => {
             await next();
-            console.log(`${ctx.method} ${ctx.url} ${ctx.status} "${ctx.headers['if-modified-since']}"`)
+            console.log(`${ctx.method} ${ctx.url} ${ctx.status} "${ctx.headers['if-modified-since']}" "${ctx.headers['cache-control']}"`)
         })
 
         // And the main handler
@@ -112,6 +112,17 @@ export default class UnboxApp {
 
         ctx.set('Cache-Control', `max-age=1`)
 
+        const ctxFresh = () => {
+            if (this.options.nginx?.cache?.support_bypass) {
+                // ctx fresh uses https://github.com/jshttp/fresh/blob/v0.5.2/index.js#L43-L49
+                // which always honors `Cache-Control: no-cache` in the request header
+                return ctx.fresh
+            } else {
+                const modifiedSince = ctx.request.headers['if-modified-since']
+                return !!modifiedSince && modifiedSince === ctx.response.headers['last-modified']
+            }
+        }
+
         // Front page
         if (request_path === '/') {
             // Allow the IF Archive admins to direct us to update the index as soon as it changes
@@ -211,7 +222,7 @@ export default class UnboxApp {
             // Send and check the Last-Modified/If-Modified-Since headers
             ctx.status = 200
             ctx.lastModified = new Date(details.date)
-            if (ctx.fresh) {
+            if (ctxFresh()) {
                 ctx.status = 304
                 return
             }
@@ -372,7 +383,7 @@ export default class UnboxApp {
         // Send and check the Last-Modified/If-Modified-Since headers
         ctx.status = 200
         ctx.lastModified = new Date(details.date)
-        if (ctx.fresh) {
+        if (ctxFresh()) {
             ctx.status = 304
             return
         }