Add HeaderDecoder with custom header name matching

[paulpdaniels]

Closes #315

Adds a new set of decoderWith operators to the HeaderDecoder and HeaderCodec companion objects. The old method decoder method now delegates to decoderWith but uses the default equality check for strings.

This allows users to override how the strings are compared when trying to determine the order of the headers in the mapping, e.g. when comparing the strings in a case insensitive manner.

[nrinaudo]

Right, this is really something I should put in the contribution guidelines. There's no issue that I can see with your PR, aside from a purely cosmetic one: the code is not formatted according to the project's guidelines.

You just need to run the scalafmtAll SBT task and it'll fix the problem(s).

[nrinaudo]

Also: given that your use case is likely quite a common one, I wouldn't be against pre-defined helpers. We have one for case-sensitive headers, why not:

  [#
  def caseInsensitiveDecoder[[#A1: CellDecoder#], R]([#f1: String#])(f: ([#A1#]) => R): HeaderDecoder[R] =
    decoderWith([#f1#])(_.equalsIgnoreCase(_))(f)#
  ]

[paulpdaniels]

@nrinaudo comments addressed

[nrinaudo]

I, err... were they? I'm not as used to the Github UI as I should be, but checking out the latest version of the branch locally still has }} rather than }}} in GeneratedHeaderDecoders, which triggers compile time warnings (and probably disable some doctests, although I haven't checked).

[paulpdaniels]

Ah ok, I switched computers because scalafmt on my windows was going crazy, so I must have lost that change. Will fix

[paulpdaniels]

Ok I think they should be fixed now 🤞

[paulpdaniels]

Apologies I lost track of this and #319 until I got the notification this morning. Anyway I think I addressed your comments @nrinaudo let me know if there is anything else you want on this.