System.NotSupportedException : Specified method is not supported: String>.Add(String item)

[matuklong]

When using Query() and Form() diferent from 'everything' results the error: Specified method is not supported: String>.Add(String item)

Test case to simulate the error in DocTest.cs:

            var url = "https://micro.prismic.io/api";
            Api api = await TestHelper.GetApi(url);
            var form = api.Form("doc").Ref(api.Master);
            var query = form.Query(
                Predicates.At("document.type", "docchapter"),
                Predicates.Fulltext("my.docchapter.title", "Using with other projects")
                );

Stacktrace:

String>.Add(String item)
SearchForm.Set(String field, String value) line 126
SearchForm.Query(String q) line 321
SearchForm.Query(IPredicate[] predicates) line 346

The main problem is within the StringValues.Add() that is not supported, besides it implement the ICollection interface. In Form.cs we can use StringValues.Concat(data[field], value); instead of .Add() to concatenate the values. I´ll submit a PR with the tests cases and the correction.

The proposed change will be:

            public SearchForm Set(string field, string value)
            {
                if (value == null)
                    return this;

                if (!form.Fields.TryGetValue(field, out var fieldDesc))
                    throw new ArgumentException("Unknown field", nameof(field));

                if (fieldDesc.IsMultiple)
                {
                    if (data.ContainsKey(field))
                    {
                        data[field] = StringValues.Concat(data[field], value);
                    }
                    else
                    {
                        data[field] = new StringValues(value);
                    }
                }
                else
                {
                    data[field] = new StringValues(value);
                }
                return this;
            }