SHRequest works only for GET authenticated requests

https://github.com/seivan/SHRequest/blob/develop/SHRequest/SHRequest.m#L58

```obj-c
  NSString *authorizationHeader = OAuthorizationHeader(self.currentRequest.URL,
                                                       [self toStringForRequestMethod:self.requestMethod],
                                                       self.bodyData,
```
but self.requestMethod is never assigned in 
https://github.com/seivan/SHRequest/blob/develop/SHRequest/SHRequest.m#L25
and the OAuth signature was invalid as it was created with "GET" even if you set a different method.