Explicitly enabling versionKey throws exception

[sebastian-nowak]

Mongoose throws exception when versionKey is explicitly set to true. It assumes it's a new name for the version key.

new mongoose.Schema({
...
}, { versionKey: true });

And here's the exception:

TypeError: Invalid `path`. Must be either string or array                                                                                                                                                
    at Object.exports.set (/tmp/TestProject/node_modules/mpath/lib/index.js:97:11)                                                                                                            
    at Object.exports.setValue (/tmp/TestProject/node_modules/mongoose/lib/utils.js:600:9)                                                                                                    
    at model.Document.setValue (/tmp/TestProject/node_modules/mongoose/lib/document.js:788:9)                                                                                                 
    at model.Model.$__version (/tmp/TestProject/node_modules/mongoose/lib/model.js:578:19)                                                                                                    
    at model.Model.$__handleSave (/tmp/TestProject/node_modules/mongoose/lib/model.js:129:10)                                                                                                 
    at model.Model.$__save (/tmp/TestProject/node_modules/mongoose/lib/model.js:189:9)                                                                                                        
    at /tmp/TestProject/node_modules/mongoose/lib/model.js:288:11                                                                                                                             
    at new Promise.ES6 (/tmp/TestProject/node_modules/mongoose/lib/promise.js:45:3)                                                                                                           
    at model.Model.save (/tmp/TestProject/node_modules/mongoose/lib/model.js:287:10)                                                                                                          
    at model._done (/tmp/TestProject/node_modules/hooks-fixed/hooks.js:101:24)                                                                                                                
    at _next (/tmp/TestProject/node_modules/hooks-fixed/hooks.js:64:28)                                                                                                                       
    at fnWrapper (/tmp/TestProject/node_modules/hooks-fixed/hooks.js:186:18)                                                                                                                  
    at model.Object.defineProperty.value.fn (/tmp/TestProject/node_modules/mongoose/lib/schema.js:250:9)                                                                                      
    at _next (/tmp/TestProject/node_modules/hooks-fixed/hooks.js:62:30)                                                                                                                       
    at fnWrapper (/tmp/TestProject/node_modules/hooks-fixed/hooks.js:186:18)                                                                                                                  
    at /tmp/TestProject/node_modules/mongoose/lib/schema.js:234:13

The docs aren't really clear about the versionKey property and only mention that setting it to false disables versioning, but in my opinion, the above behavior is completely counterintuitive.

[vkarpov15]

The docs will now specify that versionKey expects a string, and treats 'false' as a special case.