Flushing the buffer when calling IOStream.read_until_close()

Ran into this when calling IOStream.read_until_close() after calling
IOStream.read_until("\r\n"), leaving data in the buffer.
Added test_streaming_callback_with_data_in_buffer to iostream_test.py
Python3 compatibility for test_streaming_callback_with_data_in_buffer
Merge pull request #686 from omribahumi/master

Flushing the buffer when calling IOStream.read_until_close()