fix: set reply's default charset to utf8

docs/Reference/Reply.md

@@ -261,6 +261,7 @@ Sets the content type for the response. This is a shortcut for
 ```js
 reply.type('text/html')
 ```
+When set `Content-Type` with json type, if you don't set `charset`, it will use `utf-8` by default.
 
 ### .serializer(func)
 <a id="serializer"></a>

lib/reply.js

@@ -158,22 +158,14 @@ Reply.prototype.send = function (payload) {
     if (hasContentType === false) {
       this[kReplyHeaders]['content-type'] = CONTENT_TYPE.JSON
     } else {
-      // If hasContentType === true, we have a JSON mimetype
+      // If user doesn't set charset, we will set charset to utf-8
       if (contentType.indexOf('charset') === -1) {
-        // If we have simply application/json instead of a custom json mimetype
-        if (contentType.indexOf('/json') > -1) {
-          this[kReplyHeaders]['content-type'] = CONTENT_TYPE.JSON
+        const customContentType = contentType.trim()
+        if (customContentType.lastIndexOf(';') === customContentType.length - 1) {
+          // custom content-type is ended with ';'
+          this[kReplyHeaders]['content-type'] = `${customContentType} charset=utf-8`
         } else {
-          const currContentType = this[kReplyHeaders]['content-type']
-          // We extract the custom mimetype part (e.g. 'hal+' from 'application/hal+json')
-          const customJsonType = currContentType.substring(
-            currContentType.indexOf('/'),
-            currContentType.indexOf('json') + 4
-          )
-
-          // We ensure we set the header to the proper JSON content-type if necessary
-          // (e.g. 'application/hal+json' instead of 'application/json')
-          this[kReplyHeaders]['content-type'] = CONTENT_TYPE.JSON.replace('/json', customJsonType)
+          this[kReplyHeaders]['content-type'] = `${customContentType}; charset=utf-8`
         }
       }
     }

test/internals/reply.test.js

@@ -729,6 +729,30 @@ test('non-string with content type application/json SHOULD be serialized as json
   })
 })
 
+test('non-string with custom json\'s content-type SHOULD be serialized as json', t => {
+  t.plan(4)
+
+  const fastify = require('../..')()
+
+  fastify.get('/', function (req, reply) {
+    reply.type('application/json; version=2; ').send({ key: 'hello world!' })
+  })
+
+  fastify.listen({ port: 0 }, err => {
+    t.error(err)
+    fastify.server.unref()
+
+    sget({
+      method: 'GET',
+      url: 'http://localhost:' + fastify.server.address().port
+    }, (err, response, body) => {
+      t.error(err)
+      t.equal(response.headers['content-type'], 'application/json; version=2; charset=utf-8')
+      t.same(body.toString(), JSON.stringify({ key: 'hello world!' }))
+    })
+  })
+})
+
 test('non-string with custom json content type SHOULD be serialized as json', t => {
   t.plan(16)