revive lost to_utf8 method but move it into '<mapnik/unicode.hpp>'

include/mapnik/unicode.hpp

@@ -45,6 +45,10 @@ public:
 private:
     UConverter * conv_;
 };
+
+// convinience method
+void to_utf8(mapnik::value_unicode_string const& input, std::string & target);
+
 }
 
 #endif // MAPNIK_UNICODE_HPP

src/unicode.cpp

@@ -60,4 +60,12 @@ transcoder::~transcoder()
 {
     if (conv_) ucnv_close(conv_);
 }
+
+
+void to_utf8(mapnik::value_unicode_string const& input, std::string & target)
+{
+    target.clear(); // mimic previous target.assign(...) semantics
+    input.toUTF8String(target); // this appends to target
+}
+
 }