Improve containsValue Method for CharSequence Comparison

[alidandach]

Problem

The current implementation of the containsValue method checks if a provided value is an instance of String and then compares this value to the entries within headers using String.equals(). This approach limits the comparison strictly to String instances and does not account for other CharSequence implementations like StringBuilder or StringBuffer, which might also represent valid sequences of characters to be compared.

Solution

This MR introduces an improvement to the containsValue method, enabling it to handle comparisons with any objects implementing the CharSequence interface, not just String. This is achieved by converting both the value and the entry value to String before comparison, ensuring that the comparison logic remains effective while becoming more flexible.

Changes Made

• Check if value is an instance of CharSequence instead of strictly String.
• Convert value to String (if it's a CharSequence) before the comparison.
• Update the comparison logic to ensure it correctly handles CharSequence implementations.

[bclozel]

I think this change would break the implementation here. While netty headers can indeed contain CharSequence keys and values, the adapter here is only exposing a MultiValueMap<String, String>.

This change would make the following behavior possible, which I think would be inconsistent:

HttpHeaders nettyHeaders = HttpHeaders.newHeaders();
nettyHeaders.add(new StringBuilder().append("test"), new StringBuilder().append("spring"));
Netty5HeadersAdapter adapter = new Netty5HeadersAdapter(nettyHeaders);

// we got a List<String> here
assertThat(adapter.getFirst("test")).isInstanceOf(String.class).isEqualTo("spring");

// this is true, you can get a String "spring"
assertThat(adapter.containsValue("spring")).isTrue();
// this should be false, you will never get a `StringBuilder` value from the map
assertThat(adapter.containsValue(new StringBuilder().append("spring"))).isFalse();