Add test to show URL Transformer bug

This adds a test for a problem I've run into related to diacritics in NSStrings that I'm attempting to transform to NSURLs using the predefined NSURL transformer.

(Be gentle on my test case, I haven't used Specta/Expecta before but will now, it's great!)

The [URLValueTransformer included with Mantle](https://github.com/github/Mantle/blob/master/Mantle/NSValueTransformer%2BMTLPredefinedTransformerAdditions.m#L26) simply takes an NSString and passes it into NSURL's -URLWithString:. However NSURL has a [note](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSURL_Class/Reference/Reference.html#//apple_ref/doc/uid/20000301-BAJBBDIB) 
```
"This method expects URLString to contain any necessary percent escape codes,
which are ‘:’, ‘/’, ‘%’, ‘#’, ‘;’, and ‘@’. Note that ‘%’ escapes are translated via UTF-8."
```
So this will fail in the case I have provided, and simply return a nil value.

How should this be handled?

In my case, passing the string through **-stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding** will result in a successful transformation, but does that cover all cases?