Preserves body_params *and params resolved by Plug.Parsers

[riccardomanfrin]

I'm proposing the use of an additional config key (defaulted to false)

config :open_api_spex, conn_private_body_decode: true

to move the casted body result into the :private fields of the Plug.Conn.

While preserving the backward compat, it feels more inline with Plug.Conn doc and preserves the parsers result for inspection

Thanks

[mbuhot]

See also #376

[riccardomanfrin]

See also #376

didn't want to be invasive, but yes in this sense I forgot the params looking at his changes. If direction is ok and acceptable I can include those in a second round patch

[riccardomanfrin]

I've added also the params. Credits to #376

[mbuhot]

Closing in favour of #425