fix: skip URL rewriting for non-relation field values

[Nayte91]

Problem

Fixes #96.

A ?fields= query that selects scalar fields returns corrupted values:

• plain strings are percent-encoded — "Book 1" → "Book%201", "Roland Garros 2019" → "Roland%20Garros%202019";
• numbers are coerced to strings — 2019 → "2019" (a float is even truncated).

The bug only affects the query form (?fields=), not the header form (Fields:), which is why it surfaced with parametrized queries.

Root cause

In Apply, the relation handler is invoked for every scalar leaf of the document — including pure field projections that are not relations. Each such value is parsed with url.Parse and re-serialized with url.URL.String(), which percent-encodes the path; numbers are additionally re-marshalled from a string. Only the query form rewrites the body (urlRewriter + u.String()), so only it exposes the corruption.

Fix

Guard the handler so a node that is not a relation — no relation to push (preload) and no child selector to propagate — is returned untouched, before any URL machinery runs:

if !n.preload && !n.hasChildren(preload) && !n.hasChildren(fields) {
    return ""
}

The query-vs-header propagation logic is left intact; the guard only restores the precondition that block always assumed — that it operates on relations.

Efficiency

Because the guard short-circuits before url.Parse, the query re-encoding and the json.Marshal, it also removes wasted work on the fields path. Benchmark on a ?fields= query selecting 7 scalar fields (5 runs, Go 1.26):

Metric	Before	After	Δ
Time	~17,600 ns/op	~12,785 ns/op	−27%
Memory	14,220 B/op	11,512 B/op	−19%
Allocations	175 allocs/op	122 allocs/op	−30%

Benchmark used

func BenchmarkApplyFieldsQuery(b *testing.B) {
	v := New()
	body := []byte(`{"a":"Roland Garros 2019","b":"Internationaux de France de tennis","c":"A good long description here","d":"Some more textual content","e":"Yet another value with spaces","f":2019,"g":42,"h":"short","i":"value","j":"tail"}`)
	headers := http.Header{"Content-Type": []string{"application/json"}}

	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		req := httptest.NewRequest("GET", "/", nil)
		req.URL.RawQuery = `fields="/a","/b","/c","/d","/e","/f","/g"`
		rw := httptest.NewRecorder()
		if _, err := v.Apply(req, rw, bytes.NewReader(body), headers.Clone()); err != nil {
			b.Fatal(err)
		}
	}
}

Tests

Two regression tests added in server_test.go (a numeric year field was added to the JSON-LD fixture to cover type preservation):

• TestFieldsQueryDoesNotEncodePlainValues — "Book 1" stays "Book 1";
• TestFieldsQueryPreservesNumberType — 2019 stays a JSON number.

[CLAassistant]

All committers have signed the CLA.

[dunglas]

Good catch! Thanks.