Fix Netty 4.2.7 compatibility by converting HttpHeaderNames constants to strings

The netty-codec-http dependency was updated from 4.2.6.Final to 4.2.7.Final.
This introduced an incompatibility where HttpHeaderNames and HttpHeaderValues
constants (which return AsciiString) need to be explicitly converted to String
when passed to AsyncHttpClient's setHeader method.

Changes:
- Convert HttpHeaderNames.CONTENT_TYPE to string in HttpPostSink and KairosDbSink
- Convert HttpHeaderNames.CONTENT_ENCODING to string in HttpPostSink
- Convert HttpHeaderValues.GZIP to string in HttpPostSink

This ensures compatibility with AsyncHttpClient 3.0.3 and Netty 4.2.7.Final.

Author: claude

src/main/java/com/arpnetworking/tsdcore/sinks/HttpPostSink.java

@@ -127,12 +127,12 @@ protected RequestInfo createRequest(final AsyncHttpClient client, final byte[] s
 
         final RequestBuilder requestBuilder = new RequestBuilder()
                 .setUri(getAysncHttpClientUri())
-                .setHeader(HttpHeaderNames.CONTENT_TYPE, MediaTypes.APPLICATION_JSON.toString())
+                .setHeader(HttpHeaderNames.CONTENT_TYPE.toString(), MediaTypes.APPLICATION_JSON.toString())
                 .setBody(bodyData)
                 .setMethod(HttpConstants.Methods.POST);
 
         if (_enableCompression) {
-            requestBuilder.setHeader(HttpHeaderNames.CONTENT_ENCODING, HttpHeaderValues.GZIP);
+            requestBuilder.setHeader(HttpHeaderNames.CONTENT_ENCODING.toString(), HttpHeaderValues.GZIP.toString());
         }
 
         return new RequestInfo(requestBuilder.build(), serializedData.length, bodyData.length);

src/main/java/com/arpnetworking/tsdcore/sinks/KairosDbSink.java

@@ -177,7 +177,7 @@ protected RequestInfo createRequest(final AsyncHttpClient client, final byte[] s
 
             final RequestBuilder requestBuilder = new RequestBuilder()
                     .setUri(getAysncHttpClientUri())
-                    .setHeader(HttpHeaderNames.CONTENT_TYPE, "application/gzip")
+                    .setHeader(HttpHeaderNames.CONTENT_TYPE.toString(), "application/gzip")
                     .setBody(bodyData)
                     .setMethod(HttpConstants.Methods.POST);