Breaking change 45871: Drop Content-Length:0 header from GET/HEAD requests

[Alexander Aprelev]

We propose to not to include Content-Length header in GET/HEAD http requests when its value is 0(when there is no payload as part of the http request).

See official breaking change request: github.com/dart-lang/sdk/issues/45871

Please add comments to the issue if you have concerns about this change.