Fix a bug where the response cache could be corrupted.

When streaming a response, we copy data from our buffer to the cached file
on disk. Unfortunately we were copying N bytes from the front of the buffer
when we wanted N bytes from the back of the buffer.

Typically these are the same, but certain access patterns can cause them
to be different, corrpting the cached file on disk.

This was uncovered by migrating the cache tests from operating on
HttpURLConnection's API to our new API.
Merge pull request #1102 from square/jwilson_1025_fix_cache_bug

Fix a bug where the response cache could be corrupted.