fix broken install after #292 fix (#303)

The added hpp file was not added to the makefile include HEADERS, and
is therefore not installed.

Author: stuart-byma

src/Makefile.am

@@ -21,7 +21,7 @@ METASOURCES = AUTO
 lib_LTLIBRARIES = libhttpserver.la
 libhttpserver_la_SOURCES = string_utilities.cpp webserver.cpp http_utils.cpp file_info.cpp http_request.cpp http_response.cpp string_response.cpp basic_auth_fail_response.cpp digest_auth_fail_response.cpp deferred_response.cpp file_response.cpp http_resource.cpp details/http_endpoint.cpp
 noinst_HEADERS = httpserver/string_utilities.hpp httpserver/details/modded_request.hpp gettext.h
-nobase_include_HEADERS = httpserver.hpp httpserver/create_webserver.hpp httpserver/webserver.hpp httpserver/http_utils.hpp httpserver/file_info.hpp httpserver/details/http_endpoint.hpp httpserver/http_request.hpp httpserver/http_response.hpp httpserver/http_resource.hpp httpserver/string_response.hpp httpserver/basic_auth_fail_response.hpp httpserver/digest_auth_fail_response.hpp httpserver/deferred_response.hpp httpserver/file_response.hpp
+nobase_include_HEADERS = httpserver.hpp httpserver/create_webserver.hpp httpserver/webserver.hpp httpserver/http_utils.hpp httpserver/file_info.hpp httpserver/details/http_endpoint.hpp httpserver/http_request.hpp httpserver/http_response.hpp httpserver/http_resource.hpp httpserver/string_response.hpp httpserver/basic_auth_fail_response.hpp httpserver/digest_auth_fail_response.hpp httpserver/deferred_response.hpp httpserver/file_response.hpp httpserver/http_arg_value.hpp
 
 AM_CXXFLAGS += -fPIC -Wall