Only look-up Cache-Control once (dotnet#23329)

Only look-up the Cache-Control header once, rather than twice if non-empty.

Author: martincostello

src/Middleware/ResponseCaching/src/ResponseCachingPolicyProvider.cs

@@ -34,11 +34,12 @@ public virtual bool AttemptResponseCaching(ResponseCachingContext context)
         public virtual bool AllowCacheLookup(ResponseCachingContext context)
         {
             var requestHeaders = context.HttpContext.Request.Headers;
+            var cacheControl = requestHeaders[HeaderNames.CacheControl];
 
             // Verify request cache-control parameters
-            if (!StringValues.IsNullOrEmpty(requestHeaders[HeaderNames.CacheControl]))
+            if (!StringValues.IsNullOrEmpty(cacheControl))
             {
-                if (HeaderUtilities.ContainsCacheDirective(requestHeaders[HeaderNames.CacheControl], CacheControlHeaderValue.NoCacheString))
+                if (HeaderUtilities.ContainsCacheDirective(cacheControl, CacheControlHeaderValue.NoCacheString))
                 {
                     context.Logger.RequestWithNoCacheNotCacheable();
                     return false;