Summary

It seems a subtle bug to me that HurlStack will allow duplicated/conflicting HTTP Headers (specifically, the "Content-Type" header) from being added, this PR eliminate this possibility.

Illustration

The following code will generate HTTP request that have conflicting "Content-Type" headers using the current version of Volley, which will result an HTTP 400 response from the server most of the time:

public class PostRequest extends Request<String> {

  @Override
  public String getBodyContentType() {
    return "text/plain";
  }

  @Override
  public Map<String, String> getHeaders() throws AuthFailureError {
    Map<String, String> headers = super.getHeaders();
    headers.put("Content-Type", "application/json");
    return headers;
  }

  // other codes ...
}

Assumptions

I assume that Volley intends to support unique HTTP Headers only in requests, because the return type of getHeaders() is Map<String, String> instead of (the more generic) Map<String, List<String>>.

Justifications

• With the assumptions mentioned, the "normal scenarios" will still work as before, but this change prevents users from accidentally creating duplicated/conflicting HTTP Headers that will almost certainly fail.
• If we check the code of (now deprecated) HttpClientStack, it does't have this problem (it in the end calls setHeaders() instead of addHeader() in performRequest()), which is the same behaviour as this change applied to HurlStack.
• Adding "Content-Type" header is a common task for programmers, so this can potentially reduce some troubleshooting efforts for those who unknowingly tripped this error.

Background

I encountered this issue while using the swagger generated Android client (I believe this issue swagger-api/swagger-codegen#5575 describes exactly what I faced).