Fix const correctness on http Response object

A number of methods in http::Response were not marked const when they
should've been.  This is generally not an issue, as most usages of
Response are in a non-const context, but as we start using const
Response objects more, we need to be more careful about const
correctness.

Tested: Unit tests pass.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I8b31e71b6594d9328f106e1367084db42b783b6c
diff --git a/http/http_response.hpp b/http/http_response.hpp
index 6c842d7..d06c65d 100644
--- a/http/http_response.hpp
+++ b/http/http_response.hpp
@@ -71,7 +71,7 @@
         stringResponse->result(v);
     }
 
-    boost::beast::http::status result()
+    boost::beast::http::status result() const
     {
         return stringResponse->result();
     }
@@ -81,7 +81,7 @@
         return stringResponse->result_int();
     }
 
-    std::string_view reason()
+    std::string_view reason() const
     {
         return stringResponse->reason();
     }
@@ -101,7 +101,7 @@
         stringResponse->keep_alive(k);
     }
 
-    bool keepAlive()
+    bool keepAlive() const
     {
         return stringResponse->keep_alive();
     }
@@ -151,7 +151,7 @@
         }
     }
 
-    bool isAlive()
+    bool isAlive() const
     {
         return isAliveHelper && isAliveHelper();
     }