Workaround for Kestrel bleeding AsyncLocal across requests

• Workaround for Kestrel bleeding AsyncLocal across requests (#9)
• Reworded registration exception for clarity

DI small cleanup

Backported changes from dotnet/aspnetcore#10598 by @alefranz

Removed MessageHandler DI registration as not needed.
Fixed two parameters XML descriptions.

Configuration per client, replaced namespace

• Configuration per client
• Replaced namespace to simplify migration to original package when moving to ASP.NET Core 3.0
• Updated README
• Added acknowledgements section

Improved usability

Ported AspNetCore #9793 (#2)

Improved usability

Handle content headers

• Handle content headers.
• Avoid array allocation on the common scenario of a single value for the header.

Port of dotnet/aspnetcore#9370