--json with no data sets Content-Type as well

Closes #137

httpie/client.py

@@ -46,7 +46,7 @@ def get_requests_kwargs(args):
     auto_json = args.data and not args.form
     if args.json or auto_json:
         implicit_headers['Accept'] = 'application/json'
-        if args.data:
+        if args.json or (auto_json and args.data):
             implicit_headers['Content-Type'] = JSON
 
         if isinstance(args.data, dict):

tests/tests.py

@@ -436,7 +436,9 @@ class AutoContentTypeAndAcceptHeadersTest(BaseTestCase):
         )
         self.assertIn(OK, r)
         self.assertEqual(r.json['headers']['Accept'], 'application/json')
-        self.assertFalse(r.json['headers'].get('Content-Type'))
+        # Make sure Content-Type gets set even with no data.
+        # https://github.com/jkbr/httpie/issues/137
+        self.assertIn('application/json', r.json['headers']['Content-Type'])
 
     def test_GET_explicit_JSON_explicit_headers(self):
         r = http(