Raise error for `put_req_header(conn, "host", value)`

[cmcaine]

The host header is overridden by conn.host, so trying to set the header this way will usually not do what the user intended.

[cmcaine]

The error will only be raised during testing, as with the existing validation errors for headers.

[josevalim]

Can you please run mix format as well? Thank you!

[josevalim]

:green_heart: :blue_heart: :purple_heart: :yellow_heart: :heart:

[cmcaine]

Given that the host check now happens regardless of whether testing is turned on, I wonder if this commit should be adjusted or the names of the functions changed?

Oh, nevermind, you merged it ;)