Fix potentially flaky HTTPClientTests.testNoBytesSentOverBodyLimit

[fabianfett]

Motivation

If we call

context.write(ByteBuffer(string: "foo"), promise: nil)
context.flush()
context.close()

there is no guarantee that "foo" will be written to the network. Currently the test testNoBytesSentOverBodyLimit assumes this however.

Changes

• testNoBytesSentOverBodyLimit does not assume anymore, that bytes will be sent before a close of the connection.
• Use XCTAssertEqual to compare Equatable errors.

[Lukasa]

We should probably find a way to make this public so we can be confident that we're throwing the right error.