fix opensearch codec bug

[conorbros]

This path was never being run and I didn't notice the error until a large enough message went through.

The match std::mem::replace(&mut self.state, State::ParsingResponse) part is a bit misleading, it replaces self.state with State::ParsingResponse and returns what was there before, we want to set it back to State::ReadingBody if there is still more body to read. i.e. if src.len() < content_length

[rukai]

ah yep, that mem::replace makes correctness hard, the fix seems reasonable to me.