Allow HTTPClientRequest to be executed multiple times if body is an AsyncSequence

[dnadoba]

Motivation

AsyncSequence.makeAsyncIterator() was only called once during the creation of HTTPClientRequest.Body and would have been shared if an HTTPClientRequest is executed a second time.

Modification

• call AsyncSequence.makeAsyncIterator() for each invocation of HTTPClient.execute
• create a test which would previously fail

Result

We no longer share the AsyncIterator between different request executions. If the given AsyncSequence actually supports multiple iterators, and therefore can be correctly used by multiple request executions, depends on the concrete type though.