Add support for case-insensitivity in MethodOverride

Now the `Plug.MethodOverride` plug supports case-insensitive methods both in the
`_method` parameter and in the `X-HTTP-Method-Override` header. I also updated
the docs for this plug.
Drop support for the X-HTTP-Method-Override header in Plug.MethodOverride

Now only the `_method` parameter is supported.