Fix elmHeaderArg

[dharanii]

The argument name containing a hyphen will cause a syntax error in Elm.

e.g.

type BlahApi = "blah" :> Header "User-Agent" Text :> Get '[JSON] Blah

Before:

getBlah : String -> Http.Request (Blah)
getBlah header_User-Agent =
    Http.request
        { method =
            "GET"
        , headers =
            [ Http.header "User-Agent" header_User-Agent
            ]

After:

getBlah : String -> Http.Request (Blah)
getBlah header_User_Agent =
    Http.request
        { method =
            "GET"
        , headers =
            [ Http.header "User-Agent" header_User_Agent
            ]

[mattjbray]

Thanks!