fix bug

hutool-http/src/test/java/org/dromara/hutool/http/client/Issue3658Test.java

@@ -0,0 +1,24 @@
+package org.dromara.hutool.http.client;
+
+import org.dromara.hutool.core.collection.CollUtil;
+import org.dromara.hutool.http.meta.HeaderName;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.Collection;
+import java.util.Collections;
+
+public class Issue3658Test {
+
+	@Test
+	public void name() {
+		// 请注意这里一定使用带form参数的get方法，参数随便设什么都行，url也随便
+		final Request request = Request.of("https://timor.tech/api/holiday/year/2020-02")
+			.form(Collections.singletonMap("11", "22"));
+
+		final Collection<String> collection = request.headers().get(HeaderName.CONTENT_TYPE.getValue());
+		final String s = CollUtil.get(collection, 0);
+		Assertions.assertEquals("application/x-www-form-urlencoded;charset=UTF-8", s);
+	}
+
+}