fix bug

hutool-http/src/main/java/org/dromara/hutool/http/client/body/HttpBody.java

@@ -14,6 +14,7 @@ package org.dromara.hutool.http.client.body;
 
 import org.dromara.hutool.core.io.stream.FastByteArrayOutputStream;
 import org.dromara.hutool.core.io.IoUtil;
+import org.dromara.hutool.core.text.StrUtil;
 
 import java.io.InputStream;
 import java.io.OutputStream;
@@ -32,23 +33,39 @@ public interface HttpBody {
 	void write(OutputStream out);
 
 	/**
-	 * 获取Content-Type
+	 * 获取Content-Type<br>
+	 * 根据实现不同，Content-Type可能包含编码信息，也可以不包含
 	 *
 	 * @return Content-Type值
 	 */
 	String getContentType();
 
 	/**
-	 * 获取指定编码的Content-Type，类似于：application/json;charset=UTF-8
+	 * 获取指定编码的Content-Type，类似于：application/json;charset=UTF-8<br>
+	 * 如果{@link #getContentType()} 已经包含编码信息，则编码信息一致直接返回，否则替换为指定编码。
+	 *
 	 * @param charset 编码
 	 * @return Content-Type
+	 * @see #getContentType()
 	 */
 	default String getContentType(final Charset charset) {
-		final String contentType = getContentType();
+		String contentType = getContentType();
 		if (null == contentType) {
 			return null;
 		}
 
+		final String charsetName = charset.name();
+		if (StrUtil.endWithIgnoreCase(contentType, charsetName) || StrUtil.containsIgnoreCase(contentType, "boundary=")) {
+			// 已经包含编码信息，且编码一致，无需再次添加
+			// multipart无需添加charset
+			return contentType;
+		}
+
+		if (StrUtil.containsIgnoreCase(contentType, ";charset=")) {
+			// 已经包含编码信息，但编码不一致，需要替换
+			contentType = StrUtil.subBefore(contentType, ";charset=", true);
+		}
+
 		return contentType + ";charset=" + charset.name();
 	}
 

hutool-http/src/main/java/org/dromara/hutool/http/client/body/MultipartBody.java

@@ -84,6 +84,12 @@ public class MultipartBody extends FormBody<MultipartBody> {
 		return CONTENT_TYPE_MULTIPART_PREFIX + boundary;
 	}
 
+	@Override
+	public String getContentType(final Charset charset) {
+		// multipart的Content-Type头指定"Content-Type; boundary=XXXX"，编码无效
+		return getContentType();
+	}
+
 	/**
 	 * 写出Multiparty数据，不关闭流
 	 *

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);
+	}
+
+}