Use safer HeaderDictionaryExtensions

[jtkech]

While trying donet 8 new analyzers suggest to not use .Headers.Add() but .Headers.Append(), the first is a collection method that may throw if the key already exists, the second is a IHeaderDictionary extension that checks if the key already exists and if so merge the provided value to the existing StringValues.

[hishamco]

Why it's safer?

[jtkech]

I saw it while testing dotnet 8 preview where new analyzers warn on it, and then I was not able to compile. In fact the warning was in ModularTenantContainerMiddleware on.

httpContext.Response.Headers.Add(HeaderNames.RetryAfter, "10");

So we add at a given key a string that is implicitly converted to a StringValues (may hold multiple values), this by using the IDictionary.Add() method that may throw if the key already exists.

By using

httpContext.Response.Headers.Append(HeaderNames.RetryAfter, "10");

We use an HeaderDictionaryExtensions helper that checks if the key already exists and if so retrieves the existing StringValues and just add to it the new provided string.

In this file there was no warning because Headers.Add(header) uses another interface method, the ICollection.Add() method to add a KeyValuePair<string, StringValues>, but this method also fails if the key already exists, so I applied the same change.