Avoid overwriting references from users

[Genbox]

SqidOptions is provided by the user. Adding/removing items or overwriting the reference can have unintended side effects in the user's application. In this PR I simply create a local HashSet, copy over the user's data (while skipping invalid entries) and convert it to a local array at the end.

[aradalvand]

Sorry for the delay, @Genbox. Thanks for the PR.

We are not actually mutating the HashSet instance the user gives us; the very first thing we do is we replace options.BlockList with a new HashSet instance while copying over elements from the user-provided HashSet, and then modifying this new HashSet:

sqids-dotnet/src/Sqids/SqidsEncoder.cs

Lines 99 to 100 in 29559ec

	options.BlockList = new HashSet<string>(
	options.BlockList,

So, I'm not sure what the problem is here?

[aradalvand]

Ah I see what you mean now, you're talking about us mutating the SqidsOptions instance. That's valid.
Your implementation is also better in that it results in a single iteration over the user-provided HashSet, as opposed to two (under the hood) iterations in the current implementation (once by the HashSet constructor and then once more by .RemoveWhere()). Strange that I'd missed that!

[aradalvand]

@Genbox Thank you!