Use something closer to the appropriate mime-type.

This is necessary for a C client to talk to the Java server.

Technically, we should be using application/grpc+proto, but since the
"proto" portion is marshaller-dependent, we will need more plumbing
before we can use the complete form.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=77271620

core/src/main/java/com/google/net/stubby/newtransport/HttpUtil.java

@@ -23,7 +23,7 @@ public final class HttpUtil {
   /**
    * Content-Type used for GRPC-over-HTTP/2.
    */
-  public static final String CONTENT_TYPE_PROTORPC = "application/protorpc";
+  public static final String CONTENT_TYPE_PROTORPC = "application/grpc";
 
   /**
    * The HTTP method used for GRPC requests.