Remove "const" from std::map key

Updated:
    src/metdata.hpp: Remove explicit "const" from key of std::map<>
       because the key is implicitly const already (see
       http://en.cppreference.com/w/cpp/container/map and
       http://www.cplusplus.com/reference/map/map/).

On some platforms (such as z/OS UNIX System Services) explicitly
declaring the map key as "const" causes template expansion errors
as it tries to create separate allocators for "const const std::string"
and "const std::string" only to find that they clash.  (Presumably
some compilers collapse these into one earlier.)

There are no template expansion errors if the map key is left to be
implicitly const.

src/metadata.hpp

@@ -31,7 +31,7 @@ namespace zmq
     {
         public:
 
-            typedef std::map <const std::string, const std::string> dict_t;
+            typedef std::map <std::string, const std::string> dict_t;
 
             metadata_t (const dict_t &dict);
             virtual ~metadata_t ();