feat: improve / fix deserialization by parsing type String

* added api_helper.dart for a helper function.
* defaultApiClient is now a variable instead of a static field inside
  ApiClient
* a lot of functions inside ApiClient are no longer static.
* optional params are now named params  (needed to introduce a
  justIgnoreFlag as hack)
* queryParams now support the multi format and are therefore no longer a
  Map<String, String>, but a List<QueryParam>
* renamed apiException.mustache to api_exception.mustache to conform
  with other file names.
* removed unused import: 'dart:html'
* removed 'package:crypto/crypto.dart' dependency.  'dart:convert' has a
  base64 converter now.
* use null-aware operator for apiClient assignment in xxxApi
  constructors.
* enable testStoreApi (which returned a Future nobody waited for)
* fix types in tests.  Some ids were passed as Strings instead of ints.
* adapt tests to use the optional named arguments (for optional query
  args)
* generate random ids in tests.  Otherwise insertion will always succeed
  if the test has been called once.

modules/swagger-codegen/src/main/resources/dart/api_helper.mustache

@@ -0,0 +1,33 @@
+part of api;
+
+const _delimiters = const {'csv': ',', 'ssv': ' ', 'tsv': '\t', 'pipes': '|'};
+
+// port from Java version
+List<QueryParam> _convertParametersForCollectionFormat(
+  String collectionFormat, String name, dynamic value) {
+  var params = [];
+
+  // preconditions
+  if (name == null || name.isEmpty || value == null) return params;
+
+  if (value is! List) {
+    params.add(new QueryParam(name, value as String));
+    return params;
+  }
+
+  List<String> values = value as List<String>;
+
+  // get the collection format
+  collectionFormat = (collectionFormat == null || collectionFormat.isEmpty)
+                     ? "csv"
+                     : collectionFormat; // default: csv
+
+  if (collectionFormat == "multi") {
+    return values.map((v) => new QueryParam(name, v));
+  }
+
+  String delimiter = _delimiters[collectionFormat] ?? ",";
+
+  params.add(new QueryParam(name, values.join(delimiter)));
+  return params;
+}