Fix get_option_value() for strings with spaces

Problem is that

istream& operator>> (istream& is, char* str );

according to C++ documentation "Ends extraction when the
next character is either a valid whitespace or a null character,
or if the End-Of-File is reached."

So if the parameter value is a string with spaces the currently
used instruction 'ss >> ret;' copies the chars only up to the first
white space and not the whole string.

Use a specialization of get_option_value() to fix this corner case.

Bug reported by xiaozhi

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/ucioption.cpp

@@ -169,6 +169,18 @@ namespace {
       return ret;
   }
 
+  // Specialization for std::string where instruction 'ss >> ret;'
+  // would erroneusly tokenize a string with spaces.
+
+  template<>
+  string get_option_value<string>(const string& optionName) {
+
+      if (options.find(optionName) == options.end())
+          return string();
+
+      return options[optionName].currentValue;
+  }
+
 }
 
 ////