Allow overriding content-type of plaintext response factory

src/http/response/StringResponseFactory.ts

@@ -13,6 +13,8 @@ export function plaintext(value: string): StringResponseFactory {
  * Response factory that renders a given string as the response in plaintext.
  */
 export class StringResponseFactory extends ResponseFactory {
+    protected targetContentType = 'text/plain'
+
     constructor(
         /** The string to write as the body. */
         public readonly value: string,
@@ -22,8 +24,14 @@ export class StringResponseFactory extends ResponseFactory {
 
     public async write(request: Request): Promise<Request> {
         request = await super.write(request)
-        request.response.setHeader('Content-Type', 'text/plain')
+        request.response.setHeader('Content-Type', this.targetContentType)
         request.response.body = this.value
         return request
     }
+
+    /** Override the content type of the string. */
+    public contentType(type: string): this {
+        this.targetContentType = type
+        return this
+    }
 }