fix erase field

fixes #1828
closes #1829

test/beast/http/fields.cpp

@@ -988,6 +988,21 @@ public:
         BEAST_EXPECT(res[field::transfer_encoding] == "chunked, foo");
     }
 
+    void
+    testIssue1828()
+    {
+        beast::http::fields req;
+        req.insert("abc", "1");
+        req.insert("abc", "2");
+        req.insert("abc", "3");
+        BEAST_EXPECT(req.count("abc") == 3);
+        auto iter = req.find("abc");
+        BEAST_EXPECT(iter->value() == "1");
+        req.insert("abc", "4");
+        req.erase(iter);
+        BEAST_EXPECT(req.count("abc") == 3);
+    }
+
     void
     run() override
     {
@@ -1002,6 +1017,8 @@ public:
         testKeepAlive();
         testContentLength();
         testChunked();
+
+        testIssue1828();
     }
 };