Write form data without charset parameter

This commit updates the content-type for form data to not include the
charset, unless it is different from the default charset.

Some servers don't handle charset parameter and the spec is unclear.

See gh-31781

spring-web/src/test/kotlin/org/springframework/web/client/support/KotlinRestTemplateHttpServiceProxyTests.kt

@@ -136,8 +136,7 @@ class KotlinRestTemplateHttpServiceProxyTests {
         testService.postForm(map)
 
         val request = server.takeRequest()
-        assertThat(request.headers["Content-Type"])
-                .isEqualTo("application/x-www-form-urlencoded;charset=UTF-8")
+        assertThat(request.headers["Content-Type"]).isEqualTo("application/x-www-form-urlencoded")
         assertThat(request.body.readUtf8()).isEqualTo("param1=value+1&param2=value+2")
     }