fix: Restore correct functionToString behavior for updated fill method

Author: kamilogorek

packages/core/src/integrations/functiontostring.ts

@@ -21,7 +21,7 @@ export class FunctionToString implements Integration {
     originalFunctionToString = Function.prototype.toString;
 
     Function.prototype.toString = function(this: WrappedFunction, ...args: any[]): string {
-      const context = this.__sentry__ ? this.__sentry_original__ : this;
+      const context = this.__sentry_original__ || this;
       // tslint:disable-next-line:no-unsafe-any
       return originalFunctionToString.apply(context, args);
     };

packages/core/test/lib/integrations/functiontostring.test.ts

@@ -0,0 +1,25 @@
+import { fill } from '../../../../utils/src/object';
+import { FunctionToString } from '../../../src/integrations/functiontostring';
+
+describe('FunctionToString', () => {
+  it('it works as expected', () => {
+    const foo = {
+      bar(wat: boolean): boolean {
+        return wat;
+      },
+    };
+    const originalFunction = foo.bar.toString();
+    fill(foo, 'bar', function wat(whatever: boolean): () => void {
+      return function watwat(): boolean {
+        return whatever;
+      };
+    });
+
+    expect(foo.bar.toString()).not.toBe(originalFunction);
+
+    const fts = new FunctionToString();
+    fts.setupOnce();
+
+    expect(foo.bar.toString()).toBe(originalFunction);
+  });
+});