exception: catch by reference

`catch` should always be done by reference to avoid object slicing and
excess copy-constructor calls.  Preference is for exceptions to also
be caught 'const'.  (Both of these come from the C++ Core Guidelines)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7753abbef218a87ceecbd068245c14f413fadabc
diff --git a/src/security_headers_middleware_test.cpp b/src/security_headers_middleware_test.cpp
index f5cd3d5..80df725 100644
--- a/src/security_headers_middleware_test.cpp
+++ b/src/security_headers_middleware_test.cpp
@@ -33,7 +33,7 @@
                 c.close();
                 break;
             }
-            catch (std::exception e)
+            catch (const std::exception& e)
             {
                 // do nothing.  We expect this to fail while the server is
                 // starting up