fix: parse un-exploded query param to map (#101)

This fixes an issue where query parameters
that are objects with additional properties
cannot be parsed into their values. For example,
a query parameter like `tags=foo,bar,bim,baz` should
end up becoming:

```go
map[string]string{
  "foo": "bar",
  "bim": "baz",
}
```

This adheres to the [spec](https://swagger.io/docs/specification/v3_0/serialization/#query-parameters)
for paramters of style=form and explode=false.

Fixes: oapi-codegen/oapi-codegen#698

Signed-off-by: Alex Dulin <alex@morningconsult.com>

Author: alexdulin

bindparam.go

@@ -508,7 +508,7 @@ func BindQueryParameterWithOptions(style string, explode bool, required bool, pa
 			} else {
 				err = bindSplitPartsToDestinationArray(parts, output)
 			}
-		case reflect.Struct:
+		case reflect.Struct, reflect.Map:
 			// Some struct types (e.g. types.Date, time.Time) are scalar values
 			// that should be bound from a single string, not decomposed as
 			// key-value objects. Detect these via the Binder and

bindparam_test.go

@@ -542,6 +542,21 @@ func TestBindQueryParameter(t *testing.T) {
 		assert.Equal(t, types.Date{Time: time.Date(2023, 6, 15, 0, 0, 0, 0, time.UTC)}, *params.StartDate)
 	})
 
+	t.Run("map_form_no_explode_optional", func(t *testing.T) {
+		want := map[string]string{
+			"foo": "bar",
+			"bim": "baz",
+		}
+		queryParams := url.Values{
+			"tags": {"foo,bar,bim,baz"},
+		}
+		var got map[string]string
+		err := BindQueryParameter("form", false, false, "tags", queryParams, &got)
+		assert.NoError(t, err)
+		require.NotNil(t, got)
+		assert.Equal(t, want, got)
+	})
+
 	t.Run("optional", func(t *testing.T) {
 		queryParams := url.Values{
 			"time":   {"2020-12-09T16:09:53+00:00"},