`HTTP::Response::Body#each` and `BUFFER_SIZE`

[andyundso]

Hello!

The readpartial method, which each uses allows to overwrite the size of the chunks (defaults to BUFFER_SIZE). Would you accept a PR where the size can also be passed to each?

On a similar note: Right now, the BUFFER_SIZE is configured to 16KB. I tried to figure out how this value came to be, since 16KB is not a lot, but was not really able to find an answer in the code. Is there any downside to increasing this value to e.g. 4 MB?

Background: We provide an API in one of our applications where we stream large files from a 3rd party API using your gem and ActionController::Live. These files tend to be couple of gigabytes in size, so only sending 16 KB at a time results in many requests.