Minor enhancements to rate limiting information

If rate limiting information hasn't been retrieved already, it will be checked
rather than just assuming 5000 requests remaining of 5000.

Add an additional attribute `rate_limiting_resettime` to reflect github's new
X-RateLimit-Reset header. This integer value is a unix timestamp.

RateLimiting unit test updated accordingly.

github/Requester.py

@@ -87,7 +87,8 @@ class Requester:
             self.__connectionClass = self.__httpConnectionClass
         else:
             assert False, "Unknown URL scheme"
-        self.rate_limiting = (5000, 5000)
+        self.rate_limiting = (-1, -1)
+        self.rate_limiting_resettime = 0
         self.FIX_REPO_GET_GIT_REF = True
         self.per_page = per_page
 
@@ -175,6 +176,8 @@ class Requester:
 
         if "x-ratelimit-remaining" in responseHeaders and "x-ratelimit-limit" in responseHeaders:
             self.rate_limiting = (int(responseHeaders["x-ratelimit-remaining"]), int(responseHeaders["x-ratelimit-limit"]))
+        if "x-ratelimit-reset" in responseHeaders:
+            self.rate_limiting_resettime = int(responseHeaders["x-ratelimit-reset"])
 
         if "x-oauth-scopes" in responseHeaders:
             self.oauth_scopes = responseHeaders["x-oauth-scopes"].split(", ")