For S3 Key Head requests, just return the real body for now so that key length works. Closes #131.

moto/s3/responses.py

@@ -346,7 +346,7 @@ class ResponseObject(object):
         if key:
             headers.update(key.metadata)
             headers.update(key.response_dict)
-            return 200, headers, ""
+            return 200, headers, key.value
         else:
             return 404, headers, ""