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.