ClientDefaultHeaders and Headers type mismatch

[deadmann]

ClientDefaultHeaders: is of type List<KeyValuePair<string,IEnumerable<string>>>
headers: is of type IEnumerable<KeyValuePair<string, IEnumerable<string>>>

It will be nice, if both of these type can be match.

[deadmann]

this change, may break integrity with projects that are already using it... we should at least target a more important, and major version with bigger changes, before plane to change this travail issue.