Remove Range header after Seek to origin

[flobernd]

When reading data using ReadAt() and seeking to the origin using Seek(0, 0) afterwards, the old Range header is still included in the request. This causes any subsequent Read() operation to start at the offset previously passed to ReadAt().

The commit in this PR fixes that behaviour.

[klauspost]

@flobernd Thanks for the contribution. This seems to trigger some tests failures.

=== RUN   TestSetHeader
Error:     get-options_test.go:55: Test 5: Expected range header 'bytes=0-0', but got ''

This seems functionally different. bytes=0-0 would return 1 byte. So this should be investigated, if the test is wrong or the change.

=== RUN   TestGetObjectCore
Error:     core_test.go:206: Error: GetObject read shorter bytes before reaching EOF, want 946344, got 1

This also seems something that should be double checked.

[flobernd]

@klauspost Let's try a different way :-)

[klauspost]

LGTM