Treat std::string characters always as signed char

even on architectures where char is unsigned.

Fixes missing testsuite exceptions.

--- a/src/value.cpp

+++ b/src/value.cpp

@@ -352,13 +352,13 @@ namespace Exiv2 {

     long StringValueBase::toLong(long n) const

     {

         ok_ = true;

-        return value_.at(n);

+        return static_cast<long>(static_cast<signed char>(value_.at(n)));

     }

     float StringValueBase::toFloat(long n) const

     {

         ok_ = true;

-        return value_.at(n);

+        return static_cast<float>(static_cast<signed char>(value_.at(n)));

     }

     Rational StringValueBase::toRational(long n) const