Renames variable for better readability.

Renames the `customHeader` variable to `custom_header` for improved code readability and consistency.

Author: kwach

test/test.cc

@@ -8031,10 +8031,10 @@ TEST(MultipartFormDataTest, AccessPartHeaders) {
     ASSERT_EQ("text2", it->second.content);
     auto &headers = it->second.headers;
     ASSERT_EQ(3, headers.size());
-    auto customHeader = headers.find("x-whatever");
-    ASSERT_TRUE(customHeader != headers.end());
-    ASSERT_NE("customvalue", customHeader->second);
-    ASSERT_EQ("CustomValue", customHeader->second);
+    auto custom_header = headers.find("x-whatever");
+    ASSERT_TRUE(custom_header != headers.end());
+    ASSERT_NE("customvalue", custom_header->second);
+    ASSERT_EQ("CustomValue", custom_header->second);
     ASSERT_TRUE(headers.find("X-Test") == headers.end()); // text1 header
 
     handled = true;