Don't fail when blob already uploaded

[laanwj]

When the blob was already uploaded by the same pubkey, don't treat this as a fatal error. Return the blob descriptor again, but without increasing reference count.

Closes #12.

i've run cargo fmt and cargo test.

[laanwj]

Huh, can't explain the CI failure, this is what i get locally

running 12 tests
test handlers::tests::has_blob_handler_test ... ok
test handlers::tests::upload_blob_handler_test ... ok
test utilities::validation::tests::test_validate_auth_event_expired ... ok
test utilities::validation::tests::test_validate_auth_event_incorrect_action ... ok
test utilities::validation::tests::test_validate_auth_event_invalid_created_at ... ok
test utilities::validation::tests::test_validate_auth_event_invalid_kind ... ok
test utilities::validation::tests::test_validate_auth_event_missing_action_tag ... ok
test utilities::validation::tests::test_validate_auth_event_missing_expiration_tag ... ok
test utilities::validation::tests::test_validate_auth_event_valid ... ok
test handlers::tests::get_blob_handler_test ... ok
test handlers::tests::delete_blob_handler_test ... ok
test handlers::tests::list_blobs_handler_test ... ok

test result: ok. 12 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.05s

[0xtrr]

I have no idea why the tests failed either, worked fine on my machine and now it works fine when I re-run the action. If this continues to happen, I'll debug it further.