fix filtering content-type and content-length headers in HttpClientBa…

[ashashev]

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• Check if the project compiles by running sbt compile
• Verify docs compilation by running sbt compileDocs
• Check if tests pass by running sbt test
• Format code by running sbt scalafmt

Problems

The java HttpClient prohibits to add Content-Type header to request, it's reason HttpClientBacked filters this header. But the check is case-sensitive. So passing content-type (in lower case) header occurs IllegalArgumentException.

Another problem is duplicating content-type header inside request if the header name passed in lowercase.

These problems might happen if an application gets these headers from an incoming HTTP request and then pass them to a new HTTP request to another service.

[adamw]

Thank you!