support 304 status code
Fix EOF handling on rewrite_response.

When rewrite_response reads response until EOF, it exits loop with rv = APR_EOF.
But the error handing code around ON_EXIT treats APR_EOF as a failure.
Since raeding EOF is not an error, this commit sets rv = APR_SUCCESS when exit the loop.
Merge pull request #7 from tfukaya/support-304-PR

support 304 status code