Before I mistakenly thought that missing content-length meant zero when
it actually means to stream until the connection is closed.

Now the respond() function accepts transfer_encoding which can be left
as default (use content.len for content-length), set to none which makes
it omit the content-length, or chunked, which makes it format the
response as a chunked transfer even though the server has the entire
contents already buffered.

The echo-content tests are moved from test/standalone/http.zig to the
standard library where they are actually run.