Don't save settings values at module load time

Previously, we'd "cache" the RATELIMIT_ENABLE and CACHE_PREFIX settings
at module load time. That makes it harder to create tests that change
these values.

This fixes that by changing the code so we're not caching the settings
values.

Fixes #38

ratelimit/helpers.py

@@ -7,9 +7,6 @@
 
 __all__ = ['is_ratelimited']
 
-RATELIMIT_ENABLE = getattr(settings, 'RATELIMIT_ENABLE', True)
-CACHE_PREFIX = getattr(settings, 'RATELIMIT_CACHE_PREFIX', 'rl:')
-
 _PERIODS = {
     's': 1,
     'm': 60,
@@ -53,7 +50,8 @@ def _get_keys(request, ip=True, field=None, keyfuncs=None):
             keyfuncs = [keyfuncs]
         for k in keyfuncs:
             keys.append(k(request))
-    return [CACHE_PREFIX + k for k in keys]
+    prefix = getattr(settings, 'CACHE_PREFIX', 'rl:')
+    return [prefix + k for k in keys]
 
 
 def _incr(cache, keys, timeout=60):
@@ -86,8 +84,9 @@ def is_ratelimited(request, increment=False, ip=True, method=['POST'],
     cache = get_cache(cache)
 
     request.limited = getattr(request, 'limited', False)
-    if (not request.limited and RATELIMIT_ENABLE and
-            _method_match(request, method)):
+    if (not request.limited
+            and getattr(settings, 'RATELIMIT_ENABLE', True)
+            and _method_match(request, method)):
         _keys = _get_keys(request, ip, field, keys)
         if increment:
             counts = _incr(cache, _keys, period)