Keep blank values query string values on request history

We've already added empty query string matching to the matcher logic, we
should also use this when looking at request history.

Related: #101

requests_mock/request.py

@@ -110,7 +110,7 @@ def query(self):
     @property
     def qs(self):
         if self._qs is None:
-            self._qs = urlparse.parse_qs(self.query)
+            self._qs = urlparse.parse_qs(self.query, keep_blank_values=True)
 
         return self._qs
 

tests/test_request.py

@@ -130,3 +130,7 @@ def test_hostname_default_port_https(self):
     def test_to_string(self):
         req = self.do_request(url='https://host.example.com/path')
         self.assertEqual('GET https://host.example.com/path', str(req))
+
+    def test_empty_query_string(self):
+        req = self.do_request(url='https://host.example.com/path?key')
+        self.assertEqual([''], req.qs['key'])