Fix SocketHandle const-ness bug (where did this bug come from ??)

Socket/SocketHandle.cc

@@ -78,7 +78,7 @@ prefix_ void senf::SocketBody::state(SocketStateMap & map, unsigned lod)
 // senf::detail::StateMapOrdering
 
 namespace {
-    bool contains(std::string::iterator b, std::string::iterator e, char c)
+    bool contains(std::string::const_iterator b, std::string::const_iterator e, char c)
     {
         for (; b != e; ++b)
             if (*b == c)
@@ -91,10 +91,10 @@ prefix_ bool senf::detail::StateMapOrdering::operator()(std::string const & a1,
                                                         std::string const & a2)
     const
 {
-    std::string::iterator i1 (a1.begin());
-    std::string::iterator const i1_end (a1.end());
-    std::string::iterator i2 (a2.begin());
-    std::string::iterator const i2_end (a2.end());
+    std::string::const_iterator i1 (a1.begin());
+    std::string::const_iterator const i1_end (a1.end());
+    std::string::const_iterator i2 (a2.begin());
+    std::string::const_iterator const i2_end (a2.end());
     for(; i1 != i1_end && i2 != i2_end && *i1 == *i2; ++i1, ++i2) ;
     if (i1 == i1_end) {
         if (i2 == i2_end)