Encode exp as integer in app jwt.

Fixes 401 errors in current github api due to use of floating point,
rather than integer value in app jwt.

Github, y u change api behavior? ಠ_ಠ

git_credential_github_app_auth/helper.py

@@ -8,10 +8,16 @@
 
 def app_session_for(app: AppIdentity) -> requests.Session:
     session = requests.Session()
-    session.headers.update({
+    headers = {
         "Authorization":"Bearer %s" % app.jwt(),
         "Accept": "application/vnd.github.machine-man-preview+json"
-    })
+    }
+    logger.debug(
+        f"app_session_for: "
+        f"-H 'Authorization: { headers['Authorization'] }' "
+        f"-H 'Accept: { headers['Accept'] }'"
+    )
+    session.headers.update(headers)
 
     return session
 

git_credential_github_app_auth/identity.py

@@ -81,8 +81,8 @@ def jwt(self) -> str:
         """
         issue_time = time.time() - 1
         payload = dict(
-          iat= issue_time,
-          exp= issue_time + (10 * 60),
+          iat= int(issue_time),
+          exp= int(issue_time + (10 * 60)),
           iss= self.app_id
         )