The current `Builder` methods `http2_initial_stream_window_size` and `http2_initial_connection_window_size` take `&mut self`, while every other builder method takes `self`. That breaks up the chaining of options. This patch adds two methods that configure the same option, but take `self` instead, and have an underscore suffix (so, `http2_initial_stream_window_size_`). cc #1814