🐛 fix Multipart for List<int> and List<String>

[techouse]

Adds the ability to do something like

@Post(path: 'multipart_list')
@multipart
Future<Response> postMultipartList({
  @Part('ints') required List<int> ints,
  @Part('doubles') required List<double> doubles,
  @Part('nums') required List<num> nums,
  @Part('strings') required List<String> strings,
});

final Response res = await postMultipartList(
  ints: <int>[1, 2, 3],
  doubles: <double>[1.23, -1.23, 0.0, 0.12324, 3 / 4],
  nums: <num>[1.23443534678, 0.00000000001, -34251, 0.0, 3 / 4],
  strings: <String>['lorem', 'ipsum', 'dolor', '''r237tw78re ei[04o2 ]de[qwlr;,mgrrt9ie0owp[ld;s,a.vfe[plre'q/sd;poeßšđčćž'''],
);

which will produce this payload (cURL form just for brevity)

curl -v -X POST \
	-f 'ints[0]: 1' \
	-f 'ints[1]: 2' \
	-f 'ints[2]: 3' \
	-f 'doubles[0]: 1.23' \
	-f 'doubles[1]: -1.23' \
	-f 'doubles[2]: 0.0' \
	-f 'doubles[3]: 0.12324' \
	-f 'doubles[4]: 0.75' \
	-f 'nums[0]: 1.23443534678' \
	-f 'nums[1]: 0.00000000001' \
	-f 'nums[2]: -34251' \
	-f 'nums[3]: 0.0' \
	-f 'nums[4]: 0.75' \
	-f 'strings[0]: lorem' \
	-f 'strings[1]: ipsum' \
	-f 'strings[2]: dolor' \
	-f "strings[3]: r237tw78re ei[04o2 ]de[qwlr;,mgrrt9ie0owp[ld;s,a.vfe[plre'q/sd;poeßšđčćž" \
	https://foo.dev/multipart_list"

The gist of this is to use bracket notation (field[0], field[1], field[2], ...) to overcome the limitation of MultipartRequest.fields which is a Map<String, String> and as such can not have repeated keys.

This limitation has been discussed before in dart-lang/http#277 and dart-lang/http#24.

The solution I've implemented is exactly the same as suggested here dart-lang/http#277 (comment)

---

Addresses #77, #316 and #438

[codecov]

Codecov Report

Merging #439 (49e5506) into develop (d824ded) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #439      +/-   ##
===========================================
+ Coverage    93.75%   93.80%   +0.05%     
===========================================
  Files            8        8              
  Lines          448      452       +4     
===========================================
+ Hits           420      424       +4     
  Misses          28       28              

Impacted Files	Coverage Δ
chopper/lib/src/request.dart	100.00% <100.00%> (ø)

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[techouse]

@JEuler I'd like to get your thoughts on this one, as the solution is quite opinionated, however, I see no other way of making it work. Quite frankly, I'm surprised it wasn't addressed earlier. 😅

[JEuler]

I've read your discussion on the issue and looked at the PR. As I understand, nothing can be done better, because Chopper utilizes the http package.

So, we can, of course, provide something like an option to offer the capability to join list of ints to one array... but I don't feel like we should because it is easy to do by using "stringification". So, LGTM.

[techouse]

So, we can, of course, provide something like an option to offer the capability to join list of ints to one array... but I don't feel like we should because it is easy to do by using "stringification".

Well yes, in the case of ints it's easy, but as soon as you have strings or something more complex it explodes. 🙈