@"".DPL_parametersFromQueryString crashes on iOS7

Apparently `@"".stringByRemovingPercentEncoding` returns `nil` on iOS7. The following lines:

```
NSString *key = [[pairs firstObject] DPL_stringByReplacingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
paramsDict[key] = @"";
```

crash for e.g. `customscheme:/whatever?#` for obvious reasons.
