chore: backfill missing JSON builder tests (#2721)

## What kind of change does this PR introduce?

Chore. Backfill missing JSON builder tests as requested from
#2658 (review)

## What is the current behavior?

No tests.

## What is the new behavior?

Unit tests added.

Author: xlgmokha

internal/api/shared/json_test.go

@@ -0,0 +1,57 @@
+package shared
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestJSON(t *testing.T) {
+	t.Run("Defaults to JSON", func(t *testing.T) {
+		w := httptest.NewRecorder()
+
+		json := JSON(w)
+		require.NotNil(t, json)
+		assert.Equal(t, http.StatusOK, w.Code)
+		assert.Equal(t, "application/json", w.Header().Get("Content-Type"))
+	})
+
+	t.Run("Adds a Header", func(t *testing.T) {
+		w := httptest.NewRecorder()
+
+		json := JSON(w).Header("X-Request-ID", "1")
+		require.NotNil(t, json)
+		assert.Equal(t, "1", w.Header().Get("X-Request-ID"))
+	})
+
+	t.Run("Uses a custom Content-Type", func(t *testing.T) {
+		w := httptest.NewRecorder()
+
+		json := JSON(w).ContentType("application/scim+json")
+		require.NotNil(t, json)
+		assert.Equal(t, "application/scim+json", w.Header().Get("Content-Type"))
+	})
+
+	t.Run("Writes data to the response body", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		b := []byte("hello, world")
+
+		err := JSON(w).Write(b)
+		require.NoError(t, err)
+		assert.Equal(t, "hello, world", w.Body.String())
+	})
+
+	t.Run("Serializes a type to JSON", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		type person struct {
+			Name string `json:"name"`
+		}
+
+		err := JSON(w).Send(&person{Name: "gilfoyle"})
+		require.NoError(t, err)
+		assert.Equal(t, `{"name":"gilfoyle"}`, w.Body.String())
+	})
+}