update to utilize post for api call due to IPFS protocol upgrade which disallow non-POST call

Author: wizicer

src/CoreApi/FileSystemApi.cs

@@ -194,7 +194,7 @@ internal FileSystemApi(IpfsClient ipfs)
         /// </returns>
         public Task<Stream> ReadFileAsync(string path, CancellationToken cancel = default(CancellationToken))
         {
-            return ipfs.DownloadAsync("cat", cancel, path);
+            return ipfs.PostDownloadAsync("cat", cancel, path);
         }
 
         public Task<Stream> ReadFileAsync(string path, long offset, long length = 0, CancellationToken cancel = default(CancellationToken))
@@ -207,7 +207,7 @@ internal FileSystemApi(IpfsClient ipfs)
 
             if (length == 0)
                 length = int.MaxValue; // go-ipfs only accepts int lengths
-            return ipfs.DownloadAsync("cat", cancel, path,
+            return ipfs.PostDownloadAsync("cat", cancel, path,
                 $"offset={offset}",
                 $"length={length}");
         }
@@ -255,7 +255,7 @@ internal FileSystemApi(IpfsClient ipfs)
 
         public Task<Stream> GetAsync(string path, bool compress = false, CancellationToken cancel = default(CancellationToken))
         {
-            return ipfs.DownloadAsync("get", cancel, path, $"compress={compress}");
+            return ipfs.PostDownloadAsync("get", cancel, path, $"compress={compress}");
         }
     }
 }

src/CoreApi/ObjectApi.cs

@@ -51,7 +51,7 @@ internal ObjectApi(IpfsClient ipfs)
 
         public Task<Stream> DataAsync(Cid id, CancellationToken cancel = default(CancellationToken))
         {
-            return ipfs.DownloadAsync("object/data", cancel, id);
+            return ipfs.PostDownloadAsync("object/data", cancel, id);
         }
 
         public async Task<IEnumerable<IMerkleLink>> LinksAsync(Cid id, CancellationToken cancel = default(CancellationToken))