Fix context.inspect when called on the prototype (#1012)

* Fix context.inspect when called on the prototype

Fixes #837

* Add tests

lib/context.js

@@ -25,6 +25,7 @@ const proto = module.exports = {
    */
 
   inspect() {
+    if (this === proto) return this;
     return this.toJSON();
   },
 

test/context/inspect.js

@@ -1,6 +1,7 @@
 
 'use strict';
 
+const prototype = require('../../lib/context');
 const assert = require('assert');
 const context = require('../helpers/context');
 
@@ -11,4 +12,9 @@ describe('ctx.inspect()', () => {
 
     assert.deepEqual(toJSON, ctx.inspect());
   });
+
+  // console.log(require.cache) will call prototype.inspect()
+  it('should not crash when called on the prototype', () => {
+    assert.deepEqual(prototype, prototype.inspect());
+  });
 });