Remove a few duplicate lookups

Author: stevenaw

sdk/src/Core/Amazon.Runtime/Internal/Auth/AWS4Signer.cs

@@ -280,10 +280,9 @@ private static void CleanHeaders(IDictionary<string, string> headers)
             headers.Remove(HeaderKeys.AuthorizationHeader);
             headers.Remove(HeaderKeys.XAmzContentSha256Header);
 
-            if (headers.ContainsKey(HeaderKeys.XAmzDecodedContentLengthHeader))
+            if (headers.TryGetValue(HeaderKeys.XAmzDecodedContentLengthHeader, out string decodedContentLength))
             {
-                headers[HeaderKeys.ContentLengthHeader] =
-                    headers[HeaderKeys.XAmzDecodedContentLengthHeader];
+                headers[HeaderKeys.ContentLengthHeader] = decodedContentLength;
                 headers.Remove(HeaderKeys.XAmzDecodedContentLengthHeader);
             }
         }
@@ -507,13 +506,13 @@ public static string SetRequestBodyHash(IRequest request, bool signPayload, stri
             {
                 computedContentHash = chunkedBodyHash;
 
-                if (request.Headers.ContainsKey(HeaderKeys.ContentLengthHeader))
+                if (request.Headers.TryGetValue(HeaderKeys.ContentLengthHeader, out string contentLength))
                 {
                     // Set X-Amz-Decoded-Content-Length with the true size of the data
-                    request.Headers[HeaderKeys.XAmzDecodedContentLengthHeader] = request.Headers[HeaderKeys.ContentLengthHeader];
+                    request.Headers[HeaderKeys.XAmzDecodedContentLengthHeader] = contentLength;
 
                     // Substitute the originally declared content length with the inflated length due to chunking metadata and/or trailing headers
-                    var originalContentLength = long.Parse(request.Headers[HeaderKeys.ContentLengthHeader], CultureInfo.InvariantCulture);
+                    var originalContentLength = long.Parse(contentLength, CultureInfo.InvariantCulture);
                     request.Headers[HeaderKeys.ContentLengthHeader]
                         = ChunkedUploadWrapperStream.ComputeChunkedContentLength(originalContentLength, signatureLength, request.TrailingHeaders, request.SelectedChecksum).ToString(CultureInfo.InvariantCulture);
                 }

sdk/src/Services/S3/Custom/Util/AmazonS3Util.cs

@@ -231,9 +231,9 @@ public static partial class AmazonS3Util
         /// <returns>The MIME type for the extension, or text/plain</returns>
         public static string MimeTypeFromExtension(string ext)
         {
-            if (extensionToMime.ContainsKey(ext))
+            if (extensionToMime.TryGetValue(ext, out string mimeType))
             {
-                return extensionToMime[ext];
+                return mimeType;
             }
             else
             {