Don't add routes.ts suffix for absolute routesDir paths

Fixes lukeautry#360

src/routeGeneration/routeGenerator.ts

@@ -23,7 +23,7 @@ export class RouteGenerator {
   constructor(private readonly metadata: Tsoa.Metadata, private readonly options: RoutesConfig) { }
 
   public async GenerateRoutes(middlewareTemplate: string, pathTransformer: (path: string) => string) {
-    const fileName = `${this.options.routesDir}/routes.ts`;
+    const fileName = path.isAbsolute(this.options.routesDir) ? this.options.routesDir : `${this.options.routesDir}/routes.ts`;
     const content = this.buildContent(middlewareTemplate, pathTransformer);
 
     const formatted = await tsfmt.processString(fileName, content, this.tsfmtConfig as any);