Implements Range header handling for GET requests. Fix a test.
* bzrlib/transport/http/__init__.py: (TestingHTTPRequestHandler.parse_ranges, TestingHTTPRequestHandler.send_range_content, TestingHTTPRequestHandler.get_single_range, TestingHTTPRequestHandler.get_multiple_ranges, TestingHTTPRequestHandler.do_GET): New methods. Implements the Range header handling for GET request.
* bzrlib/tests/test_transport_implementations.py: (TransportTests.test_readv): Add a test with a single range.
* bzrlib/tests/test_fetch.py: (TestHttpFetch._count_log_matches): GET can succeeds with a 206 code. (TestHttpFetch.test_weaves_are_retrieved_once): log_pattern was not used here.