Fix build on web

[janek515]

This fixes the invalid type used in rsa_web.dart causing it to not build.

[janek515]

@jerson

[nilsreichardt]

@jerson Can you publish a new version on pub.dev with this fix?

[jerson]

Yeah sorry I forgot to publish to pub.dev, will do it a few minutes

[jerson]

done, there is still this warning

* `dart analyze` found the following issue(s):
  Analyzing lib, pubspec.yaml...
  
    error - lib/web/rsa_web.dart:10:26 - The argument type 'String' can't be assigned to the parameter type 'JSAny'.  - argument_type_not_assignable
  
  1 issue found.

I may need to prepare a new version soon, but in the mid time hope this helps