8325189: Enable this-escape javac warning in java.base

Reviewed-by: alanb, erikj, naoto, smarks, ihse, joehw, lancea, weijun

src/java.base/share/classes/sun/net/www/MessageHeader.java

@@ -50,6 +50,7 @@ class MessageHeader {
         grow();
     }
 
+    @SuppressWarnings("this-escape")
     public MessageHeader (InputStream is) throws java.io.IOException {
         parseHeader(is);
     }