Http header field names must be case-insensitive

GitHub has started changing their headers from capitalized words to all lowercase.
A recent change made the header fields querying case-senstive which broke gzip content detection.
This was not caught by tests because recorded files remain unchanged.
It is also possible that WireMock is auto-capitalizing.

This fixes the case-sensitivity issue and also extends that funcionality to anyone consuming the
headers generated by ResponseInfo.

Fixes #751

Author: bitwiseman

src/main/java/org/kohsuke/github/GitHubResponse.java

@@ -12,9 +12,10 @@
 import java.net.URL;
 import java.nio.charset.StandardCharsets;
 import java.util.Collections;
-import java.util.HashMap;
+import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
+import java.util.TreeMap;
 
 import javax.annotation.CheckForNull;
 import javax.annotation.Nonnull;
@@ -206,6 +207,9 @@ interface BodyHandler<T> {
      */
     static abstract class ResponseInfo {
 
+        private static final Comparator<String> nullableCaseInsensitiveComparator = Comparator
+                .nullsFirst(String.CASE_INSENSITIVE_ORDER);
+
         private final int statusCode;
         @Nonnull
         private final GitHubRequest request;
@@ -217,7 +221,12 @@ protected ResponseInfo(@Nonnull GitHubRequest request,
                 @Nonnull Map<String, List<String>> headers) {
             this.request = request;
             this.statusCode = statusCode;
-            this.headers = Collections.unmodifiableMap(new HashMap<>(headers));
+
+            // Response header field names must be case-insensitive.
+            TreeMap<String, List<String>> caseInsensitiveMap = new TreeMap<>(nullableCaseInsensitiveComparator);
+            caseInsensitiveMap.putAll(headers);
+
+            this.headers = Collections.unmodifiableMap(caseInsensitiveMap);
         }
 
         /**

src/test/java/org/kohsuke/github/GHObjectTest.java

@@ -2,6 +2,8 @@
 
 import org.junit.Test;
 
+import java.util.Objects;
+
 import static org.hamcrest.Matchers.*;
 
 public class GHObjectTest extends org.kohsuke.github.AbstractGitHubWireMockTest {
@@ -16,5 +18,26 @@ public void test_toString() throws Exception {
         // getResponseHeaderFields is deprecated but we should not break it.
         assertThat(org.getResponseHeaderFields(), notNullValue());
         assertThat(org.getResponseHeaderFields().get("Cache-Control").get(0), is("private, max-age=60, s-maxage=60"));
+
+        // Header field names must be case-insensitive
+        assertThat(org.getResponseHeaderFields().containsKey("CacHe-ContrOl"), is(true));
+
+        // The KeySet from header fields should also be case-insensitive
+        assertThat(org.getResponseHeaderFields().keySet().contains("CacHe-ControL"), is(true));
+        assertThat(org.getResponseHeaderFields().keySet().contains("CacHe-ControL"), is(true));
+
+        assertThat(org.getResponseHeaderFields().get("cachE-cOntrol").get(0), is("private, max-age=60, s-maxage=60"));
+
+        // GitHub has started changing their headers to all lowercase.
+        // For this test we want the field names to be with mixed-case (harder to do comparison).
+        // Ensure that it remains that way, if test resources are ever refreshed.
+        boolean found = false;
+        for (String key : org.getResponseHeaderFields().keySet()) {
+            if (Objects.equals("Cache-Control", key)) {
+                found = true;
+                break;
+            }
+        }
+        assertThat("Must have the literal expected string 'Cache-Control' for header field name", found);
     }
 }