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