Adding the ability to set arbitrary headers via RestClientService

Allows for, among other things, setting auth headers for API calls.

```
def apiHeaders = ['x-api-key':'rr1qFNItyRNT5qtHatvP37m6fUcWz1']
restClientService.getAsXml(
            'https://api.example.com/api/v1/items.xml',
            1000,apiHeaders)
```  
I haven't fully built out the tests.  The related asserts could sensibly be added to the existing tests, but wan't sure whether you would prefer this or having new, specific, albeit repetitive, tests.