Adding tests for RequestHandler

Feel free to let me know if you think it's too brittle or overly complex. This was as simple as I could make it considering I just started getting into unit testing again.

    Code Coverage Report:      
      2014-04-19 11:07:20      
                               
     Summary:                  
      Classes: 33.33% (1/3)    
      Methods: 94.87% (37/39)  
      Lines:   96.32% (157/163)

    \Tumblr\API::Client
      Methods:  96.88% (31/32)   Lines:  97.89% ( 93/ 95)
    \Tumblr\API::RequestException
      Methods:  50.00% ( 1/ 2)   Lines:  73.33% ( 11/ 15)
    \Tumblr\API::RequestHandler
      Methods: 100.00% ( 5/ 5)   Lines: 100.00% ( 53/ 53)