Allow DefaultHttp2Headers to be forced to lowercase.

Motivation:
I came across an issue when I was adding/setting headers and mistakenly
used an upper case header name. When using the http2 example that ships
with Netty this was not an issue. But when working with a browser that
supports http2, in my case I was using Firefox Nightly, I'm guessing
that it interprets the response as invalid in accordance with the
specifiction
https://tools.ietf.org/html/draft-ietf-httpbis-http2-14#section-8.1.2

"However, header field names MUST be converted to lowercase prior
to their encoding in HTTP/2.  A request or response containing
uppercase header field names MUST be treated as malformed"

This PR suggests converting to lowercase to be the default.

Modifications:
Added a no-args constructor that defaults to forcing the key/name to
lowercase, and providing a second constructor to override this behaviour
if desired.

Result:
It is now possible to specify a header like this:
Http2Headers headers = new DefaultHttp2Headers(true)
    .status(new AsciiString("200"))
    .set(new AsciiString("Testing-Uppercase"), new AsciiString("some value"));

And the header written to the client will then become:
testing-uppercase:"some value"

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Headers.java

@@ -19,6 +19,31 @@ import io.netty.handler.codec.BinaryHeaders;
 import io.netty.handler.codec.DefaultBinaryHeaders;
 
 public class DefaultHttp2Headers extends DefaultBinaryHeaders implements Http2Headers {
+
+    /**
+     * Creates an instance that will convert all header names to lowercase.
+     */
+    public DefaultHttp2Headers() {
+        this(true);
+    }
+
+    /**
+     * Creates an instance that can be configured to either do header field name conversion to
+     * lowercase, or not do any conversion at all.
+     * <p>
+     *
+     * <strong>Note</strong> that setting {@code forceKeyToLower} to {@code false} can violate the
+     * <a href="https://tools.ietf.org/html/draft-ietf-httpbis-http2-15#section-8.1.2">HTTP/2 specification</a>
+     * which specifies that a request or response containing an uppercase header field MUST be treated
+     * as malformed. Only set {@code forceKeyToLower} to {@code false} if you are explicitly using lowercase
+     * header field names and want to avoid the conversion to lowercase.
+     *
+     * @param forceKeyToLower if @{code false} no header name conversion will be performed
+     */
+    public DefaultHttp2Headers(boolean forceKeyToLower) {
+        super(forceKeyToLower);
+    }
+
     @Override
     public Http2Headers add(AsciiString name, AsciiString value) {
         super.add(name, value);

codec-http2/src/test/java/io/netty/handler/codec/http2/DefaultHttp2HeadersTest.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2014 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License, version
+ * 2.0 (the "License"); you may not use this file except in compliance with the
+ * License. You may obtain a copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.netty.handler.codec.http2;
+
+import io.netty.handler.codec.AsciiString;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class DefaultHttp2HeadersTest {
+
+    private static final AsciiString NAME = new AsciiString("Test");
+    private static final AsciiString VALUE = new AsciiString("some value");
+
+    @Test
+    public void defaultLowercase() {
+        Http2Headers headers = new DefaultHttp2Headers().set(NAME, VALUE);
+        assertEquals(first(headers), NAME.toLowerCase());
+    }
+
+    @Test
+    public void caseInsensitive() {
+        Http2Headers headers = new DefaultHttp2Headers(false).set(NAME, VALUE);
+        assertEquals(first(headers), NAME);
+    }
+
+    private static AsciiString first(Http2Headers headers) {
+        return headers.names().iterator().next();
+    }
+
+}