Performances: Stop initializing large vector that we’re not going to read

[irevoire]

When streaming a payload with meilisearch-rust, we have to convert a structure implementing AsyncRead to a structure implementing Stream.
This means I have to call poll_read, which have a &[u8] in parameter.

Currently, to avoid doing anything wrong, I initialize every byte of my buffer to zero here:

meilisearch-rust/src/reqwest.rs

Line 159 in 437649f

this.buf.resize(*this.capacity, 0);

But ideally, this shouldn’t be necessary since we’re never reading these bytes before writing to it.