Flaska: Add support for appendHeaders to compliment default headers instead of completely replacing them

flaska.mjs

@@ -644,6 +644,13 @@ export class Flaska {
       nonceCacheLength: opts.nonceCacheLength || 25
     }
 
+    if (opts.appendHeaders) {
+      let appendKeys = Object.keys(opts.appendHeaders)
+      for (let key of appendKeys) {
+        options.defaultHeaders[key] = opts.appendHeaders[key]
+      }
+    }
+
     if (!options.defaultHeaders && options.nonce.length) {
       // throw error
     }

test/flaska.api.test.mjs

@@ -90,6 +90,38 @@ t.describe('#constructor', function() {
 
     assert.strictEqual(flaska._after.length, 0)
   })
+
+  t.test('should have before ready setting headers on context if appendHeaders is specified', function() {
+    const appendHeaders = {
+      'Server': 'nginx/1.16.1',
+      'Herp': 'Derp',
+    }
+    let flaska = new Flaska({
+      appendHeaders: appendHeaders,
+    }, faker)
+    assert.strictEqual(flaska._before.length, 1)
+
+    let ctx = {}
+
+    flaska._before[0](ctx)
+    
+    assert.deepEqual(
+      Object.keys(ctx.headers).sort(),
+      ['Server', 'Herp', 'X-Content-Type-Options','Content-Security-Policy','Cross-Origin-Opener-Policy','Cross-Origin-Resource-Policy','Cross-Origin-Embedder-Policy','Date'].sort()
+    )
+
+    assert.notStrictEqual(ctx.headers['Server'], 'Flaska')
+    assert.strictEqual(ctx.headers['Server'], appendHeaders.Server)
+    assert.strictEqual(ctx.headers['Herp'], 'Derp')
+    assert.strictEqual(ctx.headers['X-Content-Type-Options'], 'nosniff')
+    assert.strictEqual(ctx.headers['Content-Security-Policy'], `default-src 'self'; style-src 'self' 'unsafe-inline'; img-src * data: blob:; font-src 'self' data:; object-src 'none'; frame-ancestors 'none'`)
+    assert.strictEqual(ctx.headers['Cross-Origin-Opener-Policy'], 'same-origin')
+    assert.strictEqual(ctx.headers['Cross-Origin-Resource-Policy'], 'same-origin')
+    assert.strictEqual(ctx.headers['Cross-Origin-Embedder-Policy'], 'require-corp')
+    assert.ok(new Date(ctx.headers['Date']).getDate())
+
+    assert.strictEqual(flaska._after.length, 0)
+  })
 })
 
 t.describe('#_nonce', function() {