document that some headers can't be set by WithHeader

httpclient.go

@@ -191,6 +191,7 @@ func WithContentType(ct string) Opt {
 }
 
 // WithHeader is a client option for setting custom http header(s) for each request
+// Content-Type and Accept headers will always be overwritten by the clients ContentType setting
 func WithHeader(header http.Header) Opt {
 	return func(c *Client) error {
 		c.header = header

httpclient_test.go

@@ -118,7 +118,9 @@ func TestClient(t *testing.T) {
 
 		req, err := c.NewRequest(http.MethodGet, "/test", nil)
 		assert.Nil(t, err)
-		assert.Equal(t, req.Header, customHeader)
+		assert.Equal(t, req.Header["X-Requested-By"], []string{"test"})
+		assert.Contains(t, req.Header, "Content-Type")
+		assert.Contains(t, req.Header, "Accept")
 	})
 
 	t.Run("new client valid baseurl valid HTTP client", func(t *testing.T) {