Fix float precision (#649)

The issue is that numbers like
2.01 or 3.01 can not be precisely represented with binary floating point
numbers.

This replaces all occurrences of 'std::numeric_limits<T>::digits10 + 1' with
'std::numeric_limits<T>::max_digits10'.

Background:
Using 'std::numeric_limits<T>::digits10 + 1' is not precise enough.
Converting a 'float' into a 'string' and back to a 'float' will not always
produce the original 'float' value. To guarantee that the 'string'
representation has sufficient precision the value
'std::numeric_limits<T>::max_digits10' has to be used.

include/yaml-cpp/node/convert.h

@@ -93,7 +93,7 @@ struct convert<_Null> {
   struct convert<type> {                                                 \
     static Node encode(const type& rhs) {                                \
       std::stringstream stream;                                          \
-      stream.precision(std::numeric_limits<type>::digits10 + 1);         \
+      stream.precision(std::numeric_limits<type>::max_digits10);         \
       stream << rhs;                                                     \
       return Node(stream.str());                                         \
     }                                                                    \