Fixes the HTTP2 Adapter for Requests with Payload

[hongyiweiwu]

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I am adding
• I have updated the documentation (if necessary)
• I have run the tests and they pass

This merge request fixes / refers to the following issues:

The current HTTP2 adapter doesn't work when the request contains a payload, because it only starts sending the payload after the response header from the server has been received. However, the server won't start sending the response header until the entire payload is received from the client. Hence the connection is in a deadlock until either the client or the server times out.

This pull request fixes this issue by transmitting the entire payload body before starting to listen for responses.

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[posativ]

It will also fix #762 (endStream is no longer set to false).

[krokyze]

@wendux could you please merge this? Thanks.

[ylogx]

I've verified that the patch works for POST calls (which aren't working in the current release). This should be released on pub.dev soon.

[byteslaves]

This is available, thank you

[savy-91]

Hi! When is this going to be merged?